Natural 20
When the twenty-sided die itself shows 20, before any modifiers. Many games treat it as a special success.
A "natural" roll is the number on the die before modifiers. A natural 20 is the best a twenty-sided die can show. Many rulesets make it special, for example a critical hit on an attack. What it does depends on the ruleset.
In Fantasy RPG
Fantasy RPG: on an attack roll a natural 20 is a critical hit. On checks and saving throws it is just a high roll.