new
    Text: WholeScreen;
    // The "Text:" tag defines the variable as a TextDraw, imagine that.
    
new
    ScreenStatus [ MAX_PLAYERS ] = 0,
    // "ScreenStatus" is the opacity of the fade.
    FadeTDTimer [ MAX_PLAYERS ] = -1;
    // "FadeTDTimer" is just the timer which we can kill after the players screen is done fading.

public OnGameModeInit ( )
{
    WholeScreen = TextDrawCreate( -20.000000, 0.000000, "_" ); 
    // This creates a TextDraw which covers the entire screen.
    TextDrawUseBox( WholeScreen, 1 );
    TextDrawBoxColor( WholeScreen, 0x000000FF );
    // This creates a pure-black box over the entire TextDraw (which, remember, covers your entire screen..)
    TextDrawAlignment( WholeScreen, 0 );
    // We don't use Alignment, so let's set it to zero just to make sure it doesn't bug out.
    TextDrawBackgroundColor( WholeScreen, 0x000000FF );
    // Let's set the background color to black too!
    TextDrawFont( WholeScreen, 3 );
    // I just picked a random font for this, although the font isn't important at all.
    TextDrawLetterSize( WholeScreen, 1.000000, 52.200000 );
    // TextDrawLetterSize with a box enabled modifies the Y height of the TextDraw.
    TextDrawColor( WholeScreen, 0x000000FF );
    // TextDrawColor modifies the color of the TextDraw, imagine that!
    
    //The rest of your code OnGameModeInit.
    return true;
}

CMD:fade( playerid, params[] )
{
    if( ScreenStatus [ playerid ] == 0 )
    {
        // If their ScreenStatus is 0, that means a fade is currently not active.
        ScreenStatus [ playerid ] = 1;
        // This sets their ScreenStatus to 1, which means that as long as it's not 0, their screen is fading.
        TextDrawShowForPlayer( playerid, WholeScreen );
        // We obviously have to show the TextDraw we want to fade..
        FadeTDTimer [ playerid ] = SetTimerEx( "FadeTextdraw", 50, true, "i", playerid );
        // We assign the ID of the timer to "FadeTDTimer [ playerid ]", which is one of our player-variables!
        // SetTimerEx is just a timer which passes an argument. In this case it's our ID, which is an integer ("playerid")
    }
    else return SendClientMessage( playerid, -1, "You are already fading!" );
    return true;
}

forward FadeTextdraw( playerid );
public FadeTextdraw ( playerid )
{
    switch( ScreenStatus [ playerid ] ) // This is faster than an 'if statement'
    {
        case 1:
        {
            TextDrawBoxColor( WholeScreen, 0x000000EE );
            // EE is one of the Alpha Values of a HEX Color!
        }
        case 2:
        {
            TextDrawBoxColor( WholeScreen, 0x000000DD );
            // DD is less visible than EE, but more visible than CC.
        }
        case 3:
        {
            TextDrawBoxColor( WholeScreen, 0x000000CC );
            // CC is less visible than DD, but more visible than BB.
        }
        case 4:
        {
            TextDrawBoxColor( WholeScreen, 0x000000BB );
            // BB is less visible than CC, but more visible than AA.
        }
        case 5:
        {
            TextDrawBoxColor( WholeScreen, 0x000000AA );
            // AA is less visible than BB, but more visible than 99.
        }
        case 6:
        {
            TextDrawBoxColor( WholeScreen, 0x00000099 );
            // 99 is less visible than AA, but more visible than 88.
        }
        case 7:
        {
            TextDrawBoxColor( WholeScreen, 0x00000088 );
            // 88 is less visible than 99, but more visible than 77.
        }
        case 8:
        {
            TextDrawBoxColor( WholeScreen, 0x00000077 );
            // 77 is less visible than 88, but more visible than 66.
        }
        case 9:
        {
            TextDrawBoxColor( WholeScreen, 0x00000066 );
            // 66 is less viisble than 77, but more visible than 55.
        }
        case 10:
        {
            TextDrawBoxColor( WholeScreen, 0x00000055 );
            // 55 is less visible than 66, but more visible than 44.
        }
        case 11:
        {
            TextDrawBoxColor( WholeScreen, 0x00000044 );
            // 44 is less visible than 55, but more visible than 33.
        }
        case 12:
        {
            TextDrawBoxColor( WholeScreen, 0x00000033 );
            // 33 is less visible than 44, but more visible than 22.
        }
        case 13:
        {
            TextDrawBoxColor( WholeScreen, 0x00000022 );
            // 22 is less visible than 33, but more visible than 11.
        }
        case 14:
        {
            TextDrawBoxColor( WholeScreen, 0x00000011 );
            // 11 is less visible than 22, and 00 is completely transparent.
        }
        case 15:
        {
            TextDrawBoxColor( WholeScreen, 0x00000000 );
            // This makes the TextDraw completely transparent.
            KillTimer( FadeTDTimer [ playerid ] );
            // Remember to kill the Timer, otherwise you will have a constant 50 millisecond timer running.
            TextDrawHideForPlayer( playerid, WholeScreen );
            // Hide the transparent TextDraw.
            ScreenStatus [ playerid ] = 0;
            // Reset their Status to 0, because they're no longer fading.
            return true;
        }
    }
    
    TextDrawHideForPlayer( playerid, WholeScreen );
    TextDrawShowForPlayer( playerid, WholeScreen );
    return ScreenStatus [ playerid ] ++;
    // Every time the function is called, the TextDraw is refreshed (Hidden & then re-shown and their ScreenStatus goes up 1.
}