An underdog is someone who isn't likely to win a competition or contest. When a professional basketball player faces off against a high school student who's a foot shorter, the high schooler is the underdog.
Anyone at a disadvantage, whether in general or in a particular situation, can be called an underdog. In sports, it's common to call the team that's expected to lose in a game the underdog. This word was originally used in dogfighting around 1887, to refer to a dog defeated in a fight. A 1960s cartoon character named Underdog was an unlikely (and under-qualified) superhero dog. The show's tagline was "There's no need to fear! Underdog is here!"