The Mac has a mode that Apple calls “macOS Recovery” that you can use when you need to performance maintenance on your Mac. Apple has support documents that describes macOS Recovery for Intel-based Macs and for Apple silicon Macs.
The way to boot into Recovery on an Intel Macs is to hold down Command-R on the keyboard as the Mac boots. It’s different on Apple silicon Macs. Here’s how it’s done.
1. Turn off the Mac. When you turn it on, press the power button and hold down the button while the Mac boots.
2. The screen will show icons of the bootable storage devices connected to your Mac, plus an Options icon. Select Options and click Continue. This puts the Mac into Recovery mode.
3. You may need to enter a password for an administrator account that’s on the Mac.
Then you’ll go to the Recovery screen, where you can choose from a selection of apps and perform certain maintenance tasks.
Source : Macworld