Xrepeat
xrepeat and yrepeat are members of the sprite and wall structs, where they control sizes. The term "repeat" connotes "how much distance should there be before another copy of the tile were to be drawn".
The range of values is 0 to 255, inclusive.
Sprite
xrepeat and yrepeat are the x and y scale of the sprite, respectively. These are the same values that are set by the sizeat command. Thus:
setactor[THISACTOR].xrepeat 40 setactor[THISACTOR].yrepeat 40
is equivalent to
sizeat 40 40
Setting a sprite's xrepeat to 0 is almost equivalent to calling the killit command from that actor, except that unlike killit, code following the command will still run. As with killit, such an operation comes with caveats, so see that command's page for the more information.
Wall
In the wall struct, xrepeat and yrepeat are the size\stretch of the texture used (if any).