To be glum is to be sad. Glum is a word for being depressed, bummed out, or down in the dumps.
People who are glum are sometimes said to be sullen, brooding, morose, and moody. Glum folks don't smile, giggle, or laugh — and they're rarely seen holding balloons. Being glum is a little more outward-directed than just being sad. To be glum is to act sad in front of other people, almost like you want them to ask, "Why so glum?"