In Dungeons & Dragons, slots are a measure of how much ammunition or other resources a character or monster has at their disposal. Slots in 5th edition are determined by the character’s class and level, as well as their Constitution modifier.
For example, a fighter has three slots (1d6, 1d8, 1d10). A wizard has two slots (1d4, 1d6), and so on.
When a character attacks with a weapon that they are proficient with, they can use the slot value of the weapon to determine the damage roll. For example, if a fighter is wielding a wooden club and is five levels higher than the Target they are attacking, they can use their 1d8 slot to determine the damage roll.