What exactly is plagiarism?

Hello! I am in the middle of writing my second mcript and people are now asking me what my stories are about. I do not have the time to tell them everything about it, so I concise it into a short summary. I tell them about a man whose name is Lord Sython, and they will say "Oh, that's Harry Potter!" Which makes me a little frustrated, because there are a lot of books out there with 'Lords' in them. Then I go on and tell them about the different races in my book, they're your basic mid-evil kind of races, such as elves, dwarfs, and a few others I created. But as soon as I say there are wizards, people freak out saying once more. "That's Harry Potter!" It is to the point now where I am scared to actually write for the fear of plagiarism has come over me. I know there are wizards in Harry Potter and a Lord, but to be honest isn't that in every book? Narnia, Lord of the Rings, etc. I am here just to ask what you clify as plagiarism? Should I have to worry about what I write? Thank you for taking the time to read this.