When someone sneezes, you might say "Gesundheit!" It's a way to wish good health on the sneezer, similar to saying "Bless you!"
There are dozens of languages with responses for when someone sneezes, perhaps because it can be a sign that a person is getting sick. In English, the most common responses are "Bless you" or "God bless you," but in German it's often "Gesundheit!" — meaning "health." Its use in American English might be because so many German speakers came to the United States in the 1800s and 1900s and brought their own form of well-wishing with them.