A bummer is a huge disappointment. It's a bummer when you try out for a big role in a play and don't end up getting it.
The word bummer is a very casual way to describe or make note of a frustrating experience. You can say, "It was such a bummer when we missed the train," or respond to bad news by saying, "Bummer!" The current meaning is from 1960s slang, but in the 1880s a bummer was a "loafer" or "idle person," probably from the German slang word bummler, "loafer."