If you go to your Profile page, and look on the left, you'll see an option called My Streams. If you click on that, it will give you the option to create a stream.
As for what it's for, I guess the best way to find out is to learn as you create them (you get up to 5 at a time). You could use them for private use (for example, posting meme ideas for later, etc.) or you can make a topic that you think people will be interested in, and then share the link with them, and ask them to post.
ekult made a meme with a list of streams a while back, and it's been updated a couple of times. You can look through people's streams on there to get an idea of the different things people do with them.
Here's the link to his meme with the list of streams -
imgflip.com/i/l5jxq
Anyway I hope that helps.
Caption this Meme
