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A critical hit means that you roll your damage more than once, with all your usual bonuses, and add the rolls together.
Unless these components are elaborate, preparing them is a free action. If you start casting a spell but something interferes with your concentration, you must make a concentration check or lose the spell.
×4: The weapon deals quadruple damage on a critical hit.
Spells and Critical Hits A spell that requires an attack roll can score a critical hit.
For example: 19–20/×2: The weapon scores a threat on a natural roll of 19 or 20 (instead of just 20) and deals double damage on a critical hit.
18–20/×2: The weapon scores a threat on a natural roll of 18, 19, or 20 (instead of just 20) and deals double damage on a critical hit.
[Source] When you make an attack roll and get a natural 20 (the d20 shows 20), you hit regardless of your target’s Armor Class, and you have scored a “threat,” meaning the hit might be a critical hit (or “crit”).