[Keyhead]

It's the amount of characters.

pawn Код:

CMD:test(playerid, params[])
{
    new string[128]; // Defining a new string and it's length
    format(string, 128, "Hello There, This is a test string!");// You define the "String", the length which is "128" and
    //The text in the "" is the actual string you're formatting.
    SendClientMessage(playerid, -1 ,string);// You then send it to the player.
    return 1;
}

Just look at the links I gave you.