A git diff shows the changes between two files. By extendtion, a patch is a diff with more contextual information (file line numbers, etc.).
We can use git patches to easily apply changes to Blender (for testing, review, etc).
From a Pull request, under the "Files Changed" tab, on the right hand side, you'll see a "diff options" menu:
From there, you can either download a diff or a patch. Tip: It's name the diff/patch the PR number, it's a good idea to rename it to something meaningful.