I have very little clue what this means...

It only has the potential to a problem but it's best to cut it of before it starts causing issues. It goes without saying but I have no idea what Unity is trying to say.

 

It says that Unity is having problems finding your Visual Studio installation. You're using an Alpha version of Unity, so....

 

So how do I resolve this then?

 

You would want to install a non-Beta/Alpha version of Unity, they have the f in the name instead of a or b. I know the Hub makes it too easy to install an alpha release.

 

Well I would need to know how to install the correct version of Unity then.

 

The same way you installed this version?
Open Unity Hub/Installs/Add
then choose the 3rd version from the top.

 

I already mentioned, use the versions that are listed in the Hub without the a (alpha) or b (beta) in the name. You are currently using 2019.3.0a5 (alpha, not released yet).

 

Thank you! I'm currently in the process of upgrading to version 2019.1.7f1.

 

Technically it's a downgrade, but it's still a good idea to be running on a stable version instead of the Alpha. Just note that there's a good chance the project you were using won't run properly under 2019.1, as Unity doesn't deal well with running a project on an earlier version of Unity than it was associated with. So you might want to start the project over for a clean start.