I dont think im a really smart person?

I'm 22 and I've always thought I was kind of smart but when I think about it I'm not really smart at all. I use words that I think would fit the situation and I'm sure their correct but then if you ask me what the word means I can't explain it. Today someone was talking about hitler and they said that person probally doesn't even know anything about him. And I thought to myself, wow I don't know anything about him either I know he was the leader of the nazis but I don't know anything about him. What I'm trying to get at is I think I know things but I only know so little about them which doesn't make me very bright. Also I want to be smart but when it comes to learning someone can tell me something about someone or I can read about it and I can't really explain to you what I just heard or read. Like in a lecture it just doesn't stick with me and this makes me feel stupid. This even happens withh movies sometimes. my freinds and I never really have intellectual conversations but I see that they can and I just feel they grasp things much easier than me when it comes to gaining knowledge. I feel dumb What should I do?