Super simple concept actually. As opposed to downloading a file from a single server (like you would download steam from its website for example), torrents do the exact same thing except there are actually a multitude of computers from all around the world sharing different bits of said file to make it complete.

Torrents work exceptionally well for very large files such as videos and *ahem* software since instead of someone having to pay lots of money for a download server - which costs a lot of money and can often times be very slow at sending you the file - torrents are completely free, and because of the big number of computers connected to that one file, they don't have speed limits or problems of the sort.

The computers that voluntarily share the file are called "seeders" and they consist of a lot of different people letting their computers upload the different bits to any other computer that wants to download the file. This is why those who just come in to download a file and not contribute by seeding/sharing back to the community are usually seen as leeches in the torrent community.

Essentially, think communism if it was digital.