Real meaning of string theory

If you've ever used YouTube you've probably noticed the "Recommended for You" section which suggests videos based on your viewing history. Here's what it recommended to me last time:

Check out the rightmost video. Underneath is an explanation why YouTube has picked it which says (translating the Russian title) "Because you watched Superstring theory". So that's what they mean by the string theory. It explains why this is such a hot topic nowadays.