autostep controls the maximum difference in height at which a player will "step" onto a surface without needing to jump. The default value is 5120. Note that autostep applies to view/wall aligned sprites and regular sectors.
Making the value smaller allows the player to climb higher sectors and wall aligned sprites, while a larger value means the player will not be able to climb over smaller and smaller heights in sectors and sprites.
If you are making a permanent change to this value, it is best to do it from EVENT_ENTERLEVEL so the game never overwrites your value with the default.