It's a fantasy role-playing game.

DM: You are Scanlan Shorthalt, a gnome bard . You are the member of the famous group of adventurers. Previously, A breath of frost kills one of the giants before it is able to do much. Percy's engineering talent makes a showing, between a hand grenade and one of the exploding arrows he made for Vex.
DM: Okay. You do a quick little hand gesture, hum to yourself the arcane ritual, and a very large, mystical see-through replica of your fist appears in the air above you and goes spinning off in the air in an arc. Straight up Popeye-punching towards the dragon.  Let's go ahead, and Bigby-- what do you have to roll for that?
Player: Oh, I have to roll for that?
DM: Let me see what the effect is on that, remind myself real fast. Bigby's Hand. All right, so it's an AC of 20. Or no, that's the hand, all right. Clenched fist, cool. So make a melee spell attack. So roll a d20 and then add your spell attack modifier.
Player: That's the spell attack bonus thing? Okay. 19.
DM: 19. (wham) Just barely hits as the fist goes slamming into the side of the dragon's face, big old Popeye punch to the side of the jaw. So go ahead and roll 4d8 points of force damage.
Player: 4d8. I'm taking yours, and I hope that's okay. Ten, 13, 18, 21. Plus something?
DM: That's 21 points of damage, nice. (whack) It gets socked in the jaw. The hand whips around again, preparing for the next round. Do you want to use anything, move, use your bonus action?
Player: I'm 20 feet away. I will go 18 feet away from it and drop the grenade.
DM: Okay. So you move forward right here?
Player: Yeah, closer to it.
DM: Okay. The grenade is right there on the ground that Percy made. All right. Going to stay where you are?
Player: Can I keep moving?
DM: You can keep moving. You moved 18 feet backward, you mean?
Player: No, I had to move closer to it.
DM: Right, so you moved three feet of your 25 feet, dropped it, and you still have 22 feet you can move.
Player: But if I leave, he's going to whack me.
DM: No, he's this far away. You have to be within a certain range for him to hit you.
Player: Wait, how close did I put the grenade to him?
DM: It's currently 18 feet away from him.
Player: No, I wanted to put it two feet away from him.
DM: Oh, I see! There, that makes more sense. So now there is a grenade on the ground, right there. You are there, right next to Grog. So yeah, you cannot move away without it taking a swipe.
