How/when does a dragon’s breath weapon recharge?

Dragon breath weapons used to be simple: 3 uses per day. They magically recharged at either midnight or dawn, depending on your DM.

As I read through the dragon section, I see new (to me) info on Lair and Legendary actions, environmental effects, and more. Those all seem pretty straight forward. However, when I get to the actual stat blocks for dragons (for a Black Dragon) I see stuff like:

Acid Breath (Recharge 5–6).

Did I miss where this was explained? What exactly does this 5-6 mean?

How/when does a dragon’s breath weapon recharge?