A Critical Hit is an attack which does bonus damage. In addition, any status effects on the hit have a 100% proc rate (unless the target is immune to that effect).
All characters have a base of 1% 
Crit.
By default, a crit does +50% damage. Wielding a Scythe increases critical damage by 25%.
By default, only Regular Attacks can crit. The Fellblade's Versatile passive allows Skills and Spells to crit. 
Boon will allow a skill or spell to crit, even if the character does not have Versatile.