lev_comp is the NetHack special level compiler. It turns the text-format .des-files in the dat-dir into binary .lev-files which will be loaded by NetHack when making the special levels.
To compile a .des-file into a .lev-file you run the lev_comp executable with the lev-files given as parameters, and lev_comp will create the .lev-files in the directory where you ran it. Usually you want to run it in the dat-directory where the lev-files are. For example, on linux, the following command run from the dat-directory will compile all levels:
The des-file format is actually not a programming language, as NetHack always executes the different commands in the same order, eg. traps are created before monsters, which are created before objects, even if they're in a different order in the des-file.