YouTube is an online service that gives you a place to store and share all of the videos that you've created. Aside from
just giving you a place to store your videos, it also allows you to watch other videos that members of the YouTube community have created. If you choose to make your videos public then people can watch your videos too.
You can watch videos without signing up but in order to participate in the community and upload your own videos, you'll want to create an account.