Levels in Clash Mini
There are multiple types of levels in Clash Mini. You have your profile level, which is viewable from the menu at the top-left corner, there's levels for Heroes and Minis that make them stronger, then there's levels inside of matches for Minis that help them unlock the abilities unlocked from their overall level.
Your profile is leveled up from simply playing matches. The more matches to play, the higher level your account will become. There are a few features locked behind this, such as unlocking private matches at level 5, but for the most part, this won't affect your ability to play the game.
Outside of matches, Heroes and Minis have levels. You can level up any given unit by collecting duplicate copies of it. For heroes, you get shards and after getting so many shards, the hero will level up, increasing its stats and sometimes unlocking a new ability. Minis, on the other hand, level up every time you get another copy of it. After getting so many copies, minis unlock abilities as well.
In matches, Minis can be leveled up by getting duplicate copies of the same mini and combining it with an existing one on your field. This is also how you can unlock the use of abilities for your Mini that were unlocked in the main menu. Be sure to strategize which ability may be the best to unlock first in case you don't get enough copies to unlock them all!