I don't think terms necessary means "words or expressions". It is quite a common expression, although a bit formal, and it has a broader meaning. In a way is the only translation which comes to my mind at the moment.
Relate to doesn't really mean "understand" but it has a very similar meaning. I'm afraid I'm unable to give you an explanation about this. You'll have to wait for a teacher.
P.s: I'm not a teacher nor a native speaker.
Student or Learner