Question Can anyone tell me what this means? And after that, how do I fix it?

Any help is greatly appreciated.

 

I think the popup message is already self explinatory. Basically you are trying to open a very old unity project with a new unity editor version. Since each unity updates brings new features as well as deprecating old features, you have to go through a upgrade progress to upgrade the project from the old unity version to your desired version.

Pressing continue will try to automatically upgrade the project for you, but you likely need to make some modifications manually as well after upgrading.

 

Ok. Thank you!

 

other possibility is that the project is missing whole ProjectSettings/ folder,
or ProjectSettings/ProjectVersion.txt file, which contains the project version info.

(if its missing, you get that same warning)