Rolling a d20 and getting a “nat 20” or natural 20 is an a critical hit. A critical hit doubles the damage of the damage dice roll before the modifier.
Bob the paladin rolls a d20 and gets a natural 20 (critical hit!).
Bob then rolls a d8 for his weapon damage. He gets a 5. The 5 is doubled to 10. Bob then applies his +3 strength modifier to the total, for 13 damage all together.
Sidenote: Some DMs consider a natural 20 a hit even if that still doesn’t equal the creature’s armor class even with the attack modifier applied. For example, if a creature has an armor class of 30, some DMs would consider a nat 20 still a hit even if 20 + an attack modifier of 5 doesn’t equal or exceed the creature’s AC.