"Other" includes everything that is not any of the other categories: documents, apps, music, TV, iOS files... etc. It includes mail and text messages along with all their attachments: pictures, videos, audio... etc.
Insufficient space remains to do much of anything other than allow your Mac to function. If you need to create more space you have to delete stuff. Then, don't forget to empty the Trash—not only Finder Trash, but Trash for all email messages, photos... etc.
iCloud storage is fine and it's good that you have it but you have to delete things on your Mac to increase free space.
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When purchasing a Mac or an iOS device it's reasonable to ask "how much storage is enough," but there is no "average" or typical Mac user. The Mac I happen to be using at the moment is actually using about 200 GB, far exceeding the entire storage capacity of your Mac. And, I use iCloud. Whether that describes an "average Mac user" is up to you, but professional creative artists (for example) require a lot more storage than that. Terabytes, certainly.