Critical Hit
Critical Hit is a battle feature in Kingdom Hearts. It can be considered as an additional, powerful blow during the end of a combo and also during some magic abilities and possesses more damage power than the follow-up combination attacks. The magic ability Trinity Limit is rich in critical hits.
Furthermore Keychains and Sora's ability Critical Plus can help the chances of critical hits being present in a combination and also determine the strength of the critical hit. Examples of Keychains that encourage critical hits are Fairy Harp, Pumpkinhead, Olympia and the Metal Chocobo. Depending on the Keychain's Passive Ability the strength of the critical hit and its overall presence in the combo varies.
Importance
Critical Hits are important as they help with defeating bosses. Also if you are after the rearest item in the game Mystery mold then you must defeat a Black Fungus right on a critical hit, that is defeating it with a critical hit. It is easiest done using the Trinity Limit magic ability.
When in Atlantica, the easiest way to defeat a Sheltering Zone heartless is to defeat it right on a critical hit. This stops it from exploding into more Sea Neons.