This sets the number of seconds for which receipt of the most recent IGMP Membership Report for a group on an interface is considered evidence that a member is still present on that interface, per its description. It's effectively the group membership aging timer: too short a value risks prematurely dropping a still-interested receiver between its periodic reports, while too long a value keeps forwarding multicast traffic to a segment that lost its last member. A value of 260 seconds means a receiver has to be silent for over four minutes before this router considers the group membership expired on that interface.