It adjusts the interval when keepalive messages are expected on the line so that you don’t get disconnected during a large data transfer. If three keepalive intervals pass without receiving a keepalive, the connection will be dropped.
Direct from Cisco:
Enable Keepalive Updates on Virtual Asynchronous Interfaces
Keepalives are enabled on all virtual asynchronous interfaces by default. To change the keepalive timer or disable it on virtual asynchronous interfaces, perform the following task in global configuration mode:
Specify the frequency with which the Cisco IOS software sends keepalive messages to the other end of an asynchronous serial link.
vty-async keepalive [seconds]
The default interval is 10 seconds. It is adjustable in one-second increments from 0 to 32,767 seconds. To turn off keepalive updates, set the value to 0. A connection is declared down after three update intervals have passed without receiving a keepalive packet.
Virtual terminal lines have very low bandwidth. When adjusting the keepalive timer, large packets can delay the smaller keepalive packets long enough to cause the session to disconnect. You might need to experiment to determine the best value.
— at: http://www.cisco.com/en/US/products/sw/iosswrel/ps1826/products_configuration_guide_chapter09186a00800d9bc4.html