This compresses images down to 16MP and videos to 1080p (a change that’s unlikely to be noticeable for most people and purposes).
If you’ve been saving photos at their original sizes, you can free up tons of space by converting them to Google’s “Storage saver” option (which used to be called “High quality”). Unless you currently have a model 5 or earlier Pixel phone (in which case, you will, for now, keep the unlimited “Storage saver” option), as of June 1st, 2021, every photo and video backed up to Google Photos is going to count against your Google storage.