Applicability is the usefulness of something for a particular task. Hammers have great applicability for driving in nails.
When something is applicable, it is suited to something or useful for a task. The applicability of a thing refers to how useful it is in a given situation. On a resume, talking about your favorite movie probably would have no applicability: it's pointless. A plunger has great applicability in a bathroom, but no applicability anywhere else. You can think of applicability as usefulness plus timeliness. If something has applicability, it will come in handy at the moment it is needed.