I have read research paper of the professor to whom i want to apply for internship but cannot understand it fully
Professors are, or at least tend to be, busy people. So my primary advice is to keep it short and to the point. Tell them which year you are in, that you are looking for a research internship, add a very high level topic you are interested in (the more specialized a topic you insist on, the less likely a match can be found), and that you went through their papers (add names if it’s not too many), and that you enjoyed reading them. They will write back to get into any details if interested, or maybe even refer you after talking to you if they aren’t.
I do not agree with what was said here too that understanding every step of a paper is needed or even helpful. You are an undergrad seeking to learn from them by working with them, not a peer-level collaborator. Obsessing about getting every word when some will be standard tricks or conventions obvious to researchers in the field but not outside is both frustrating and wasting time you could have used more productively. What you ideally need is a good grasp of the paper’s intuition and internal logic. The professor should realize in a follow-up discussion if they feel you are prepared enough.