GraphQL allows you to not only fetch a single piece of data, but allows you to also fetch related data, ie traverse the relational graph of data. So if you have friends, and sometimes want to fetch your friend's latest posts/pictures, you can do so as you like with one single query. In REST, you would first get all your friends, then for each of these friends get all the posts/pictures as individual queries.

In short: it allows you to query a database much more naturally in one go, rather than having to fetch data, and depending on that data then fetch more data. On top of that, it's typed, relatively easy to learn and well documented. I'd highly recommend it, even if it's a tad more complicated than plain REST.