TheHistorian gives good advice; HârnWorld is a kind of short encyclopaedia of much of the setting (i.e., a little bit about a lot of things).
The sheer amount of information *can* be very daunting, even for experienced Hârn nuts. Doubly so for Hârn newbies like yourself. The thing to remember is this; you don't need to read—let alone remember—all that information. You only need to take in what you need at the time. There are hundreds of individual locations or settlements described, mapped, their inhabitants detailed, and adventure hooks compiled. But unless you intend for your players to go there, there's no reason for you to even buy it or download it. I'd recommend this same strategy for the individual kingdoms available. You probably need an understanding of what the kingdoms are all about, but the detail can wait until you need it.
But you do
need some kind of framework to figure out what's going on and where things are happening, and that's where HârnWorld comes in. It can give you that overview you need without overwhelming you. It's not exactly "Hârn For Dummies", but it's the best we have.
And remember, as the GM you have the final say. If an article about a place doesn't fit what you need then change it. It's your game, after all. The caveat is radical changes may negatively impact too much other material to be controllable. Hârn material (both canon and fanon) has been written to be internally consistent (mostly). The mass of material can take small changes easily, moderate changes pretty well (although some work will be required to maintain consistency), but big changes are going to be a problem.