What is energy and protective junk for?

You know those things in your inventory that say energy+3?

What does that really modify? I havent seen a change in my stats.

you have to equip them. If you do they add +3 to your energy...

which ones can rangers be equipped with? Some dont work.

they are equiped like a shield. since you're a ranger, if you're using a bow you can't use a focus at the same time.

You can only use foci if you meet the requirements (if any) marked on it, and only when you're using a one handed weapon. Bows are all two handed, so foci are usually not worth much to rangers.