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Babe is a slot-based, proof of Stake algorithm. It breaks time into epochs, with each epoch being broken into slots. In Polkadot, each slot is six seconds long, our target block time. BABE will select an author (or several) to author a block in each slot.

One way to assign authors to these slots would be simply to take turns. However, in a round-robin pattern, adversaries always know who the next author is and can use that information to coordinate attacks. Ideally, nobody knows who the slot author is until he or she proves it.

Each slot can have a primary and secondary author (or “slot leader”). Primary slot leaders are assigned randomly. Because the function is random, however, sometimes there are slots without a leader. In order to ensure a consistent block time, BABE uses a round-robin system to assign secondary slot leaders.

Source: https://medium.com/polkadot-network/polkadot-consensus-part-3-babe-dcc2e0dd8878