When you're sombre — or somber — you're acting glum, depressed, or sad. Being sombre is the opposite of being perky or happy.
This is a word that has to do with the mood of a person or event. A funeral is going to be sombre, and the people at the funeral are usually pretty sombre, too. It's a quiet, serious, and even gloomy kind of feeling. Objects can look sombre if they're dull and colorless. Just don't get sombre and confused by the spelling: the "re" ending is a British thing, while Americans usually end it with "er."