Round robin
Round robin is a lead routing method that assigns new leads to a group of reps in turn, so each receives a fair share of the volume.
In practice
Round robin is the usual default when no ownership or territory rule applies. Done carelessly, it sends leads to reps who are on holiday or at capacity, or splits contacts from the same account between different reps.
Apply round robin only after account matching and territory rules have run. Skip reps who are out of office, consider weighting by capacity or segment, and check the distribution regularly to confirm it is fair over the period.