What does the blinking folder with question mark mean on my MacBook Pro when booting?

in Recovery, the drive is NOT mounted, and that is EXACTLY what you need to do a deep repair. You can not make substantial corrections to the drive you re running from, back your Mac would crash.

How to repair a Mac storage device with Disk Utility - Apple Support

click on the blue text in my response above and it will link to an article with step-by-step instructions:

How to repair a Mac storage device with Disk Utility - Apple Support

How to start up from macOS Recovery - Apple Support

the blinking question mark indicates one of two things:

• I could not find the boot drive where you said it was.

Sometimes if you leave your Mac alone, it may start searching, and provided the boot drive is bootable and available, it may find it and boot from it.

• the drive is not a bootable drive.

That can happen when Directory corruption occurs, and it can not easily be fixed. The way forward is to boot into Recovery, where the disk is NOT used at all, and launch disk utility and repair the disk.

If the disk will not come clean after several attempts, the next question is:

¿what is the date of your most recent backup, and by what method?

You may need an alternate drive with MacOS on it to proceed.

"No fault found” says that under the enormously constrained conditions, in the exceptionally brief amount of time allotted, using the limited tests available, no faults were found. 

It is NOT the same as "All is Well"

But it does give you same assurance that there are not GROSS problems with the Hardware.

If your Mac starts up to a question mark - Apple Support