valid_lft forever preferred_lft forever <-- what does this mean?

Just looking at my ethernet interface.. I see this response... what does this mean...?

ipconfig...
lo: <LOOPBACK,UP,LOWER_UP> mtu 16436 qdisc noqueue
link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
inet 127.0.0.1/8 scope host lo
inet6 ::1/128 scope host
valid_lft forever preferred_lft forever

What does this mean what is it telling me ??

Thanks all !!