A multitude is a very large number or a huge crowd. If you see a multitude of zombies approaching, you're in trouble.
Sometimes the word multitude refers to the common people, or the masses — that is, everyone in a society apart from the political elite. There are a lot of words and phrases for this, like hoi polloi, which is Greek for "the many," and "the great unwashed." Those last two terms are usually used in a disapproving way, but multitudes is generally a positive term. If you were to say that a leader inspired the multitudes to rise up against their oppressor, we'd assume that you sympathized with the multitudes.