When something is incongruous, it does not fit the place, the people, or the feeling around it. The thing itself may be fine; it just sits in the wrong picture. A bright plastic toy on a grave. A joke in a very quiet, sad room. A stiff, old word in a text message to a friend. Writers and careful speakers like this word when they want to say “this does not belong here” without shouting. It is a little formal. You will see it in news and books more than you will hear it in a shop. People stretch it a little: not only for things you can see, but for ideas and pairs that do not go together. Though actually, if you only mean “strange” or “surprising,” this is the wrong word — the strangeness has to be that two things do not match, not just that one thing is new.