You must sign a Contribution License Agreement (CLA) before your PR will be merged. This a one-time requirement for Microsoft projects in GitHub. You can read more about Contribution License Agreements on Wikipedia.

However, you don't have to do this up-front. You can simply clone, fork, and submit your pull-request as usual.

When your pull-request is created, it is classified by a CLA bot. If the change is trivial (i.e. you just fixed a typo) then the PR is labelled with cla-not-required. Otherwise, it's classified as cla-required. In that case, the system will also tell you how you can sign the CLA. Once you have signed a CLA, the current and all future pull-requests will be labelled as cla-signed.

Signing the CLA might sound scary but it's actually very simple and can be done in less than a minute.