Storage space

You mean other than by deleting old/unwanted/undeeded data/files away?

Any files on disk (that you can view in the Finder) can simply be moved to the trash (in your dock).

Other applications (such as Photos and Music) store data in their internal databases, so you can delete photos/songs from within the app to free up space.

Certain apps can offload data to iCloud so that smaller versions (e.g. thumbnails, proxies, etc.) are stored on your local system with the full-sized version stored in iCloud. The OS automatically downloads the full-sized content any time you try to access it, and this can be a huge saving if you have a lot of idle/little-used data. Check System Preferences -> AppleID -> iCloud -> Optimize Mac Storage to turn this on.