When running BOSS, what exactly does this mean? Bash Tag suggestion(s): {{BASH: Relev, Delev}}

its for using http://www.gameskyrim.com/topic/1346866-relz-wrye-bash/page__hl__wrye%20smash. it is a sweet mod managing program (among many other things). the "bashed tags" are for the "bashed patch" the program creates from the mods in your load order. the bash patches are basically a merging of the leveledlist changes from your mods. this allows some mods that change the same things to be used together when previously one would overwrite the other. think of it like compatibility patch made especially for you the bashed tag suggestions from BOSS help the program create the bashed patch more effectively.

hopefully this helps. personally a bit new to using wrye bash myself. but the link provides tons of info and explains it much better then me.

Delev means that the mod deleted an item from a leveled list. Relev means the mod changed the number of items or the level of a leveled list.

EDIT: Bash's ability to merge leveled lists is one of the most important functions of any modding tool. Without it, mods that alter the same leveled lists stomp all over each other (i.e. you end up with mods that don't work right).

Too Much Information:
These refer to leveled lists, which are lists of dynamic, "random" (i.e. not statically placed) stuff. When you open a container (chest, cupboard, dead bandit, etc.) the game generally has semi-random stuff placed in that container based on a list. There are different types of lists for animals, low-level bandits, high-level bandits, etc.