Segments are lists of memory blocks which are used to organize your code. You can use them to define the order which things are placed in memory (data after code etc). Your can combine segments to form new segments and you can use modifiers to process the output of a segment. Finally, you can direct the output of a segment to a files or disks or simply throw it away.

This is implemented in Kick Assembler in a backward compatible way, so if you don't use segments, everything is placed on a default segment and directed to the standard output file as you are used to.