Function CreateExplosion create an explosion at the specified coordinates.
There is a limit as to how many explosions can be seen at once by a player. This is roughly 10.

(Float:X, Float:Y, Float:Z, type, Float:radius)
float X The X coordinate of the explosion.
float Y The X coordinate of the explosion.
float Z The X coordinate of the explosion.
int type The type of explosion.
float radius The explosion radius.

Return Values:
This function always returns 1, even when the explosion type and/or radius values are invalid.

public OnPlayerEnterCheckpoint(playerid)
    // Get the player's position
    new Float:x, Float:y, Float:z;
    GetPlayerPos(playerid, x, y, z);
    // Create an explosion at the player's position
    CreateExplosion(x, y, z, 12, 10.0);
    return 1;

Related Functions
The following functions may be useful, as they are related to this function in one way or another.