Instant reservation means that the guest can make a reservation directly through the channel, without the host having to approve the request. See below the sales channels that allow this type of reservation:

Channel
Observations
AirbnbInstant reservation is one of the options available on the channel.
BookingThe channel supports only instant reservations.
DespegarThe channel supports only instant reservations.
VrboInstant reservation is one of the options available on the channel.
ExpediaInstant reservation is one of the options available on the channel.
Homes & VillasThe channel supports only instant reservations.
HousiThe channel supports only instant reservations, but the Housi Marketplace team can intermediate the communication between guest and host for negotiations, if necessary.
Stays has a tool that allows you to set a time limit to avoid last minute reservations. If unforeseen reservations are what prevent you from connecting to channels, learn more about the feature.