It's a fantasy role-playing game.

DM: You are Vax Vax'ildan, a half-elf rogue . You are the member of the famous group of adventurers. Previously, They  also found that a number of gold coins, a bit of his horde that had been buried, were now exposed by these strange smooth tunnels that were carved into the rock. Upon  Vex , Scanlan, and Trinket inquiring there, the rest of the party scattered: Pike and Keyleth went to find what seemed to be the owner of the house, now frozen in fright in a far alcove; and the rest of the group traveling towards the mouth of the cavern itself.
DM: And you feel the ground beneath you begin to shake, as it bursts out of the ground. That's a 20 to hit.
Player: 20? Can I use Evasion or no?
DM: To reduce the damage in half if you want. Huh? What's your AC?
Player: Oh, with the boots on, 18.
DM: It rolled a 20.
Player: Yeah, I know. So tell me what happens.
DM: So, as you look around you all of a sudden the jaws start coming up out of the ground behind you, the teeth biting into your body as you try and wrest yourself away from its grip. You take 20 points of piercing damage.
Player: Oh, well, Uncanny Dodge. Right? Evasion is for spell effects--
DM: Right, Evasion does not help you in this circumstance, Uncanny Dodge allows you to reduce the damage by half. So yes, you can do that. So you take ten damage instead. I need you to make a dexterity saving throw.
Player: That's okay. All right, so. Okay. Natural 20!
DM: As the jaws attempt to clamp around you, you manage to just pull yourself away, tearing your knees across the teeth, but rolling in the opposite direction, right in the place where you were as it bursts out of the ground just beside you and rights itself around in that direction. As its full body emerges now, you can see its tail flop out of the tunnel it just made with this giant hooked stinger on the end as it whips off in your direction. That is going to be a 15 to hit.
