IsPlayerInNoBuild is used to see if a player is in a "nobuild" area. By default, a trigger_ff_script named "nobuild" will not allow players to build while the player is touching the "nobuild" trigger_ff_script entity.
IsPlayerInNoBuild( ent_id )
The ent_id passed in is simply an integer index that refers to an entities game code ENTINDEX().
The output is true or false depending if ent_id is a player and then if the player is in a "nobuild" area or not.