If you’ve ever attended a funeral, you were probably struck by how quiet, earnest, and solemn the mood was. You can use the word solemn to describe anything that's really serious and dignified.
The adjective solemn comes from the Latin sollemnis, which means formal or ceremonial. You can still use it to describe a ceremony or event, but it's also a good word for talking about someone who's serious and sincere and maybe lacks a sense of humor about certain things. Here's a trick for remembering it: Think of a "sole man," a serious guy standing alone at a party talking to no one.