Press Manage to dive deeper into how you use storage on your Mac and access macOS.
Scroll your cursor through the storage use graph to see information on how much space is used by your apps, documents, photos, system, users, and so forth. Remember that when you move a file to the Trash, its storage space doesn't become available until you empty the Trash. Go to the Apple Menu and choose About this Mac. Then expand them when you need to use them again. Note: If you’re running an older version of OS X, you may need to click More Info to find the Storage option. Click on the Apple icon and choose About This Mac. If you don't want to delete certain files, you might be able to save space by compressing them. The simplest way to check Mac hard drive space is by accessing About This Mac.
Also choose Mailbox > Erase Deleted Items. Why is System Taking So Much Disk Space on Mac.