service tcp-keepalives-in

[Steve]

Hiii all,
i've seen this recently:

R1(config)#line vty 0 4
R1(config-line)#exec-timeout 3
R1(config-line)#exit
R1(config)#service tcp-keepalives-in

I know that the exec-timeout command prevents an idle session from consuming the VTY indefinitely, but i couldn't understand what does the service tcp-keepalives-in mean?? could someone plz explain to me?
much appreciated