Proxy files are lower-bitrate copies of the camera footage used during the editing process. Because the proxy files (ProRes 422 Proxy is a common example) are in a format that PP can more easily decode you can get by with a less powerful PC. Then when you render the final output PP uses the original camera files (applying all your edits) so you won't see any quality loss in the final output. Done properly PP makes the process almost transparent. Even though I have a fairly powerful PC I still use proxy files to edit, it's just so much smoother than editing cranky H.265 directly.
A good free program to convert your camera footage to ProRes is
RockyMountains Movie Converter . Using proxy files in PP is more than I can fit in a reply but there are many good tutorials on YouTube and the Adobe site.