Ouroboros divides the time into epochs, and each epoch is divided into slots. A slot is a short period of time (eg. 20 seconds).
Credit: Cardano papers ( https://cardanodocs.com/cardano/proof-of-stake/ )
A slot has only one Slot Leader (SL):
The slot leader has a (sole) right to produce one and only one block during his slot:
The number of slot leaders is equal to the number of slots in epoch and if a slot leader missed their slot, they will lose the right to produce a block. To produce a block they need to be elected again.