You've successfully created a PR and it's in the queue to be merged. A maintainer looks at the code and asks you to rebase your PR so that they can merge it. Say what? The maintainer means that there have been other code changes on the project since you branched which means that your branch cannot be merged without conflicts and they would like to you to sort this out. These are the steps you should take.

http://akrabat.com/the-beginners-guide-to-rebasing-your-pr/