Complex sentence: An accessibility relation is a binary relation R\\,\\! between possible worlds which has very powerful uses in both the formal/theoretical aspects of modal logic as well as in its applications to things like epistemology, metaphysics, value theory, and computer science.
Split into simpler sentences: An accessibility relation is a binary relation R\\,\\! between possible worlds. This "accessibility relation," furthermore, has important uses in both the formal/theoretical aspects of modal logic as well as in its applications to things like epistemology, metaphysics, value theory, and computer science.