The purpose is to be a basic raycaster, by using a sprite as the map to be rendered on GPAGE 0,0. Line 18 is used to control the slope of the ray. Look up sprite-based raycasting on Scratch for more.
See if lines 84-102 help. ABXY aim, and L fires. X,Y is your position, and W,Z is the end point of your line(where you aim at) created by a GLINE X,Y,W,Z,RGB(255,255,255) (not shown).
Does anyone know how to make it look as though you are within a room? I want to eventually make Portal-style test chambers w/ objects, but I want to start out simpler. Please let me know.