It means:
In life, people keep leaving or entering. So, what you see is invisible; Remember, forget.
What you saw: it means that all the people you saw before are gone, and you can't see them.
Remember: some mutual friends are gone now and have been forgotten over time!
In life, there are gains and losses. Therefore, the invisible see; Forgot, remember.
Invisible, see: some things are not known until they are lost.
Forgotten, remembered: some accustomed things are suddenly lost, only to remember.
However, does invisibility mean non-existence? Remember, will it never disappear?
The above two sentences are understood clearly, so this sentence should be understood, right?