In a TTRPG, rolling the lowest possible value (usually a 1) on a die when checking if an action succeeds or fails.
Such a check is done by having one value which represents a characters skill or capability in a particular area (higher being more proficient), adding a random value, and comparing the sum to another value which represents the difficulty of the action (higher being more difficult). If the sum is greater than the difficulty, the action succeeds, otherwise it fails. Thus a greater degree of skill increases the probability of success, as one would expect.
This would imply that a character with a skill value of N would always succeed at actions of difficulty N or less, since the lowest roll possible is a 1, and N + 1 is greater than N. It is at this point that the critical failure rule comes into play:
A critical failure means the action always fails, no matter what. Period. No matter how trivial the action or how competent the character.
This ensures that player characters, no matter how skilled or powerful they become, are never able to fully insulate themselves from the game's random element.