[Threshold]

Basically, you can think of it this way, 1 = success/true, 0 = failed/false

Examples:

pawn Код:

MyGateIsOpen = 1; //It is open
if(MyGateIsOpen == 1)
{
   CloseGate;
   MyGateIsOpen = 0; //It is now closed or false (without the use of booleans)
//...

pawn Код:

public OnPlayerSpawn(playerid)
{
    return 1; //The player has spawned
}

public OnPlayerRequestClass(playerid, classid)
{
    if(classid == 12) return 0; //Do not spawn player, player spawned with class = false
    return 1; //Else let them spawn by returning 1
}

I don't know of any existing tutorials about returning values, based around 0 and 1, but keep searching if you need help with it. Or check out the SA-MP Wiki.