When something happens incongruously, 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 neon sign over an old wooden door. A laugh in a sad scene. A very casual word in a serious letter. It is the adverb for incongruous, so it tells you how something looks, sounds, or is done: out of place. It is a little formal. You will meet it more in writing than in chat. Though actually, people also use it with a small smile, when the mismatch is funny rather than wrong. It is not a word for “sadly” or “by bad luck.” It is a word for “in a way that does not belong here.”