According to macOS Help, safe mode stops some software from loading, and performs a check of the startup disk. Here’s a more detailed and accurate account of what it does.
Safe mode
From killing a process, through a regular restart, to Recovery and a bootable external recovery disk, all you need to know about fixing your Mac in macOS 11 and 12.
They contain recently browsed web pages, ARM code translated from Intel executables, security data, font information, and much more. And they can cause problems.
A guide to what you can do when a Big Sur or other macOS update goes wrong, from Safe mode to DFU mode and Configurator 2.
From Startup Manager to Internet Recovery on Intel Macs, and how to engage each of those in an M1 Mac. With comprehensive diagrams.
M1 Macs don’t use that warren of startup key combinations, but a logical structure of choices, mostly when starting up in Recovery Mode. Here are the full details.
The Mac is now in a documentation crisis. Documentation debt is leaving many Mac users floundering around in the dark.
Diagnostic, Recovery, Safe, Startup Manager, Verbose, reset SMC/NVRAM, firmware restore, Target Disk/Display, Single-User (SUM) in outline.
Disk integrity checks appear to have been dropped from Safe Boot. OpenDirectory databases are rebuilt. With a full listing of all blocked extensions.
Start up in Safe Mode: should it start by checking and repairing your boot disk, or should you do that separately in Recovery Mode?
The Eclectic Light Company 