Understanding "GPU" ?

Hey Guys and Ladies, Here's hoping your day has gone well for you. I've been reading the "REVIEWS" of the different Graphics Processing Units [ GPU ] and trying to find the common denominator of what makes a good one ???
Is it speed or being able to process many numbers into a picture or what ? Can some of you help me out on this understanding ?
Just what is it that I should be looking for and how do I know if "it" will fit in the proper place - - - not too large etc,etc. Also - - - How do you know if you only need one - - two or - - ahem - - half dozen
Keep in mind that it would be for the Flight Sims.
Appreciate your help, caveman
Is it speed or being able to process many numbers into a picture or what ? Can some of you help me out on this understanding ?
Just what is it that I should be looking for and how do I know if "it" will fit in the proper place - - - not too large etc,etc. Also - - - How do you know if you only need one - - two or - - ahem - - half dozen

Keep in mind that it would be for the Flight Sims.
Appreciate your help, caveman