Critical hit
If you hit your target while getting a (non-modified) d20 roll within your weapon's threat range, this results in a critical threat (even if your modified attack roll wouldn't beat the opponent's AC).
A critical threat means you have a chance to score a critical hit. To do so, a confirmation roll is made, with the same to-hit bonus being applied (plus extra confirmation bonuses such as Seeker, Kensei enhancements, Fighter Critical Accuracy, etc.); in this roll, 1's do not automatically fail to hit. This roll must beat the target's AC in order to "confirm" the critical hit.
In DDO, a confirmation roll can still succeed on a roll of 1; it does not auto-miss on such a roll.
If that roll also results in a hit against the target's AC, the critical hit is confirmed, and the base damage is rolled once for every critical hit multiplier. If the confirmation roll is a miss, then the attack is not a critical hit and is rolled as a normal attack (IE, auto-success on natural 20 or test vs. AC for non-20).
For instance a +2 longsword doing 1d8 + 2 damage on a normal hit will do 2d8 + 4 damage on a critical hit since it has a 2x critical hit multiplier. On another hand, a +3 greataxe doing 1d12 + 3 damage on a normal hit will do 3d12 + 9 damage on a critical hit, because it has a 3x critical hit multiplier. Your strength bonus, enhancement bonuses, and seeker bonuses are also counted as part of the base damage, and thus multiplied by the critical hit multiplier.
Additional damage is generally not rolled multiple times (such as elemental, alignment, bane, or sneak attack damage), except when specified in the weapon's effect description. (For instance burst weapons do additional elemental or alignment damage on a critical hit.) Seeker weapons will improve the second roll (to confirm the crit) and also add to the base damage before the multiplier.