Length of stay (LOS) is the number of nights in a booking. Properties often set rules around it: a minimum length of stay — say two nights on a weekend, or a week in peak season — protects against short gaps that are hard to fill, while a maximum can keep rooms free for higher-value bookings.
Average length of stay is also a useful health metric: longer average stays mean fewer arrivals and departures to service for the same occupancy, and usually a lower acquisition cost per night.
Why it matters for direct booking
On your own booking website you set length-of-stay rules directly — minimum nights by season, by room, or by date — so the stays you take are the ones that work for you, without an OTA's constraints. It is part of availability and pricing being yours to control.