How does combat work on D&D?
Let's take a look at a simple melee attack.
Basically, there is an atribute called Armor Class (or AC for short). There are many factors that contribute to AC but, for simplicity sake, let's just say it's based off the quality of armour a character is wearing.
To perform an attack, a D20 (a die with 20 sides) is rolled. That's what is called an attack roll.
There are bonuses that are added after the dice roll. Again, for simplicity sake, let's just say that it depends on how good is the attacker's weapon.
Then, the two values are compared. If the attack roll is equal to or greater than the target's Armor Class, the attack was succesful and another roll (a damage roll) is made. We weill discuss damage rolls in a future ocasion.
If the attack roll is lesser than the target's AC, then the attack fails and nothing happens (no damage).