fb files basically contain many of the files the game requires. For modding, it is easier to extract those files, edit them (i guess you might call this the modding part), then putting those files back together in one big file (the fb file).
1. you have an fb file
2. you extract game files from the fb file using fbExtractor
3. now you have many files and can edit them
4. after you edit those files you put them back into one fb file using fbBuilder
you dont need fbExtractor or fbBuilder programs, you can just use a hex editor instead, but the programs make it easier to extract/compile the files within an fb file