I understand most of what each number represents, but monster/NPC statblocks often have a number in the damage section before the damage roll in parentheses. For example, for a Sahuagin Priestess (MM, p. 264), one of her attacks looks like this:
Bite. Melee Weapon Attack: +3 to hit, reach 5 ft., one target. Hit: 3 (1d4 + 1) piercing damage.
What does the 3 in "3 (1d4 + 1)" mean?