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.