A critical doubles the damage or healing of a spell, and can be obtained from special gear. If you are level 50 or up, some gear at the bazaar has critical ratings. If you are not, some special gear from the hoard packs in the Crowns Shop has critical ratings.
First off it is called Critical... and critical means there is double the damage or healing. It usually starts at level 60 with Diego, I think but there is a posibitlity of level 50's. Sometimes you can get critical from level 40 pack gear or higher.