Skills are learned abilities of a character. The skill level shows how good the character is at that skill. Skill levels can be increased by allocating skill points earned from gaining levels. Skills can also be improved by reading specific books or completing quests within the games.
For most skills, the success of an action governed by one is determined by either directly checking skill level against a threshold or with a skill roll. Rolls are used for most intrinsically random actions like hits in combat and random encounters.
With Speech, direct checks are used as well as rolls in Fallout and Fallout 2, with direct checks completely taking over in later installments. Skills governing the success of game play actions with low degree of random chance (like lockpicking) used rolls in classic games but were likewise replaced.
A skill roll (also called "skill challenge" or "skill check") emulates the way skill checks are done in traditional pen-and-paper RPGs. A random number from 1 to 100 is generated. If it is lower or equal to the skill level, the action is a success. So, for an unmodified check, skill level is effectively a chance of success for a relevant action.
A check can have a modifier that the skill level is adjusted by for the purpose of the check. So a positive modifier makes the action easier while a negative one - harder. Combat skills use automatically calculated modifiers from e.g. the ambient darkness level. Modifiers for checks associated with specific object or logic (like a dialogue option) are set explicitly. Since skills can grow up to 300%, the hardest challenges can have modifiers of as much as -100.
Some skills also limit the success rate by 95% no matter what. A few skills can also generate critical successes and critical failures. For combat skills, the effect is determined by Critical Hit and Failure tables.
