Sirben4, Welcome to Physics Forums!
There are "old sayings" in most cultures in our world. They are old, that is, they have been around a long time and continue to be used because they contain some elements of truth. Here on PF we say, "Google is your friend". This is repeated many times here and it does contain this truth: there is a growing body of information available to us in cyberspace, and Google is one great tool to gain access to that knowledge.
So, here's what I did: in Google I used the search terms "high gradient RF structure" and in a few minutes I found just what the term means and where it's used, ect. If you do the same you will find most of your answer. Once you have done your searching and reading if you still have doubts or some further questions then post them here on PF. Members here are always ready and willing to assist all interested and curious searchers of scientific knowledge.
Cheers,
Bobbywhy