The online Collins dictionary shows that it is in the 10,000 most common words. So it is in the vocabulary of a typical middle school student.

It is a fairly common word, where I am anyway. Why would you like to know this? :)

Yep. A regular word, used/heard fairly often.

I can think of two meanings. Relish can be a fruit-based (mainly tomato-based) sauce. That makes it a common word. But it can also mean something like enthusiasm and appreciation. 'He bit into the apple with relish.' In this context I think it's a literary-register word. That is, you probably won't hear it used this way unless someone is reading aloud from a book. (not common)