>>370458795
I'm still fairly new but +1 is the level that item/skill currently is, fighting with it increases it's level, you can see the EXP of any item on the inventory by hovering over it. Turning it purple is the upgrade, and makes it idk if it's Unique but one tier above what it's at right now. Check the skill list you get with it. 
Also protip because I found this by accident: skills have the same levels, too. You gain them by upgrading and can equip/swap skills on the tree to level different skills with different modifiers. It's an ok system, kind of confusing.