Mac storage basics · Photos, music and video
What happens if I delete the Photos Library file?
Short answer: It deletes every photo, video, album and edit in it unless they exist elsewhere. Even with iCloud Photos, unsynced items would be lost — optimise, move or archive instead.
You delete every photo and video stored in it, plus albums and edits — unless they also exist elsewhere. With iCloud Photos fully synced, your pictures remain in iCloud, but anything that never finished uploading, and any library that isn't the System Photo Library, would be lost.
If you want the space back, safer options are turning on Optimise Mac Storage for iCloud Photos, moving the library to an external drive, or exporting and archiving old years first. See iCloud & iCloud Photos.