The exact path
~/Library/Application Support/MobileSync/Backup
To see the size without opening anything:
du -sh ~/Library/Application\ Support/MobileSync/Backup
Each subfolder is one backup of one device, named by a long identifier rather than anything readable. Several folders usually means several devices, or several years.
Before you delete anything. If a device is not also backed up to iCloud, this folder is the only copy of its photos, messages and settings. Check on the device — Settings → your name → iCloud → iCloud Backup — before removing anything here.
Why they accumulate
macOS keeps old backups when you back up a replaced device, and when a device is restored. Nothing prunes them automatically. A Mac that has seen three iPhones over six years can hold all of them.
The safe way to remove one
Use Finder's own device screen rather than deleting the folder by hand: connect the device, select it in the sidebar, then Manage Backups. Deleting the folder directly can leave the backup database referring to a backup that is no longer there.