Ok, so Armor skills... the #1 explained subject on literally any site that discusses Monster Hunter ><. Honestly, it's the most obscure part of Monster Hunter by far alongside damage calculations. So, all regular pieces of armor you have gives you skill points in certain skills. For this example I'll take the default Derring set that you start with (or can buy at the armorer dude, can't recall). I'll also throw lots of links to Kiranico since it's imo the best reference for MH4U.
If you take a look at the Derring Headgear while shopping, you should see something like this in the 4rd page (Skill Tree / Skill Points) :
These are the skill points that you receive from equiping that piece of armor. You will get similar information for other pieces of the sets. Now if you press Y, you'll get into your "Skill Details" screen which lists the skill points each pieces of equipment gifes you including the decorations set in their slots. Using Up/Down arrows, you can hover the skill you're interested in and press Y again. That'll popup a window that describes the skill so if you check "Mounting", you will see a screen with Mounting : Skills that make it eaasier to mount and topple monsters as well as an "Active Skills" tabloid. Those are the threshold required to unlock skills and for Mounting it looks like this :
This means that if your total reaches +10, you will get the Mounting Master skill, once it reaches +15 you get Rodeo God and if you end up with -10 or less Mounting you will get Saddle Sore.
Now, if you want to see a recap of what you have currently equipped, you can go to your status (Menu's 2nd page). On the 2nd status page, you have a list of Active Skills and their descriptions. Assuming Derringer set, you'd see Mounting Master and Recovery Spd +1. If you go on the 3rd page you will see a table similar to the one you when you were shopping except that it uses all of your equipped items instead of swapping one for whatever you were shopping.
Btw, this is a copypasta, if something sounds wrong, I can explain further and/or correct the information.