A pull request is a consistent set of changes in the code base. At any given moment in time Bitcoin Core has many open pull requests. Reviewing pull requests is very important to move forward Bitcoin.
Many pull requests are related to a specific BIP, but this is not always the case.
- List of current open pull requests for Bitcoin Core on GitHub
- How to Review Pull Requests in Bitcoin Core, by Jon Atack, regularly updated (as of 2020)