A confirmation roll is made to confirm a critical threat is a critical hit: it is a to-hit roll which gets the same bonuses and penalties as the to-hit roll that resulted in the critical threat (plus extra confirmation bonuses such as Seeker, Kensei enhancements, Fighter Critical Accuracy, etc.), and it is successful if it results in a hit (not necessarily within the weapon's threat range).
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).