Compressed is just a smaller size, that's all. It's all the same content. Just not a massive file size.
As for explaining the combat system, I don't know that I can explain the whole combat system in a single post...but, if there's specific parts of combat you have questions about, I can certainly answer them.
In short, combat takes place on a hex grid. You have facing on this hex grid. Which face you're being attacked at will determine how well you can defend against it (if even at all). HP is relatively low due to armor being the main source of protection you'll have, as it functions similar to an HP bar (reaching 0 and breaking).