Why do ranged weapons in D&D have two numbers to show their range e.g light crossbow 80/320?

You can use a ranged weapon from 5ft to it’s full range without there being a disadvantage and I know that the second number, in this case 320, is how far it can shoot, but what is the first number for? Are you at a disadvantage when you use the light crossbow cloaser than 80ft away from your target?