In D&D, most groups use a rule that states that rolling a 1 during a d20 check will result in an automatic failure, regardless of the task at hand or any modifiers that the character may have. There are also groups that will rule such a result as a "critical fumble," resulting in some detrimental effect for the character who rolled the 1. The opposite is also true, with groups treating a natural 20 on a d20 as an automatic success, regardless of how improbable the result. This is often referred to as a critical success by the fans.

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A lot of groups treat this rule as something that is part of the D&D 5e official rules, but that's not strictly true. A 1 = automatic miss and a 20 = critical hit is indeed part of the attack roll rules, and rolling a 1 on a death save will result in two failures while rolling a 20 will restore a hit point and save the character from death's door. This rule is not part of the official rules when it comes to other uses of the d20, such as ability checks or skill checks.

D&D Is Making 1 = Failure & 20 = Success Part Of The Rules

The latest Unearthed Arcana article is available on the D&D 5.5 rules update in 2024. This document presents a rule called the d20 Test, which is used to determine whether a d20 is needed. If the check would be lower than 5 or higher than 30, then it's not needed, as the task is either too easy or impossible. Under the d20 test rules, any attack roll, ability check, or saving throw now has a roll of 1 as an automatic failure and a 20 as an automatic success. Rolling a 20 also provides the character with a point of Inspiration.