The project you are opening contains compilation errors.
Entering Safe Mode allows you to resolve these errors without waiting for the rest of your project to import.
If you select Ignore (not recommended), your project will continue to import, but it may be in a broken or unusable state.

The project being opened has compilation errors.
For VRChat, this is most commonly caused by some of the curated VRChat Creator Companion packages, and is relatively harmless.
Some packages like the above may temporarily raise console errors, but resolves themselves once the rest of the project is compiled.
Hitting Enter Safe Mode lets you see what console errors the Unity Project is complaining about. If they have to do with any of the packages in the above explanation, they can typically be safely ignored, and the project can be opened normally without Safe Mode.
If you Ignore Safe Mode and check your console after clearing it, see if you have any lasting console errors that may actually break your project.