From OpenLieroX Wiki
A level defines not only how the level looks by images, but also where there are air and walls and wether or not the latter is destroyable or not. A wall can be destroyed either by firing certain weapons or digging through it. A level might also define objects in the level like doors, fans or other things.
You can find a List of levels here.
The most popular form, and the one with most effect is the level making. A level defines the walls and dirt of a level. Read the main article: Level Making