Critical Hit is a term that refers to an attack hitting "a weak spot". Such a hit causes twice the normal amount of damage. This is generally a random process. The games announce that a critical hit has landed by using the phrase, "A Critical Hit!".
Some moves, like Slash, have a higher chance of scoring a critical hit than others. There are also moves that cause an increase in the amount of critical hits dealt by a Pokémon. These include Focus Energy. The chance of causing a critical hit can be modified by using items that increase the critical hit level. Although, some Pokémon have abilities that prevent the opponent from landing a critical hit. In Generation I, critical hits were based off of speed.