Pull request
A proposed code change packaged for teammates to review, discuss, and approve before it joins the shared codebase.
What it tells you about a developer
A developer whose pull requests merge quickly and stay merged ships reliably. Their pull request descriptions and their replies to criticism are an unfiltered writing sample you can read before ever contacting them.
A pull request bundles one or more commits and opens them for discussion. Teammates comment, ask for changes, and eventually approve it. On open source projects the whole conversation stays public, so it is a readable record of how a developer collaborates.