Macintosh HD is the default name for your internal hard drive.
On the hard drive will be the macOS Mojave system and all your personal info
such as documents, photos, movies, apps etc etc.
You can get an overview of this if you click on the top left of the screen
and selecting About This Mac and click on Storage
See the screenshot below, my disk has been renamed SSD APFS.

If you could do the same and post back a screenshot as above would be helpful.
Quite often the information in Storage is wrong and can sometimes be corrected by rebuilding
the Spotlight index.
How to rebuild the Spotlight index on your Mac – Apple Support
Open System Preferences> Spotlight> Privacy
Move the Macintosh HD (or the name you gave your disk) in to the Privacy panel.
Quit System Preferences.
Open System Preferences> Spotlight> Privacy highlight Macintosh HD and press the minus button.
The mac will start rebuilding the Spotlight index.
Check by clicking on Spotlight in the menubar and enter a word, if it is indexing you should see a progress bar.
You can also use the Terminal app in your Applications> Utilities folder to force Spotlight to reindex.
Open Terminal.
Enter this command,
sudo mdutil -E <drag the icon of the drive you want to reindex here>
(make sure there is a space after the E.)
enter your Password.
(what you type will not be repeated on screen)
press Return
Terminal will confirm if indexing is enabled on the Volume/ drive and after a few seconds
Spotlight should begin reindexing.
To confirm click on the Spotlight icon in the menubar, type in a word, if there is a progress bar, Spotlight is indexing.
If re-indexing has not solved your problem then run this app, https://www.omnigroup.com/more
this will give an accurate account of the storage used. When the app has created its overview you can look at
the Users folder in the output and see what each user has stored, you can then delete files from there.
Do not delete any files or folders in any System or Library folders or any files you do not understand.
If you can get back with the results then we can see if we can move forward.
What mac, model and year do you have and which OS would you like to upgrade to?