"Tabable" refers to objects and monsters that you can target by using the "Select Next NPC" control key, which is by default bound to the Tab key. Often, people who are running through a zone will keep hitting the Tab key repeatedly to find monsters, harvestable nodes or quest objects. This is especially a good idea if you need to see from afar which monsters are Aggro towards you.
Most objects and monsters in Everquest II, including NPC, harvestables and quest gatherables are tabable. However, some objects, such as some special quest objects and corpses, are only clickable, and you will need to move your cursor over them and then click either one of your mouse keys.
Player Characters (PCs) can not be selected with the Tab key either, but they can be targeted with the Options / Controls / Targeting Keys / Select PC key, which is bound to F7 by default. Yourself, your pet, group members and group pets can be targeted with the F1, Shift-F1, F2-F6 and Shift-F2-F6 keys, correspondingly, by default.