The minimum stay is the minimum number of nights the landlord accepts per booking.
Uniplaces also sets a minimum stay. Landlords are obliged to offer a minimum of 27 nights for each booking — with the exception of Barcelona, where the minimum stay is 32 days.
Keep in mind that some landlords define a minimum stay but, in fact, prefer a longer one. For example, a landlord may require a minimum stay of 3 months, but prefer to accept booking requests from students who want to stay longer, such as a whole semester. This is why sometimes booking requests are declined — landlords prefer to wait for a student who wants to book for a longer period of time.
You can find the information regarding the minimum stay under the Rental Conditions section in every listing.