Those are Talent Grid upgrade levels.
Once a hero is maxed, you can use Class Emblems to further upgrade it.
Here’s my Wilbur at +7 — notice how 7 of the Talent Grid Nodes are unlocked.
Each hero has a set Class, and each Class has a set Talent.
The Talents are special abilities that have a chance to activate in different circumstances, and don’t require mana like the Special Skill.
The Talent Grid also allows for further upgrades to attack, defense, and health — and you have some choice in which you focus on.
Here are some other threads with useful info on this: