Any ability (or spell) that can harm an enemy is an attack. Roll the die on the attack, and deal that much damage. If you roll a 1, the attack misses and has no effect. For attacks with multiple dice, the leftmost die is called the Primary Die. It determines whether the attack is a hit or a miss.
Exploding Critical Hits. Rolling the max on a Primary Die is a critical hit (crit). When you crit, roll the Primary Die again, and add the result to the total. Repeat this each time you roll the maximum—there’s no limit, except your luck! Crits also ignore monster armor.
Rushed Attacks. A hero may attack more than once on their turn, but these additional attacks are rushed, imposing cumulative disadvantage for each additional attack after the first—increasing the chance to miss and lowering the average damage for subsequent attacks. For abilities that trigger a save (e.g. Grappling), enemies roll with increasing instances of advantage instead.