If you go to account settings and scroll down - there is the Leave SW option, people can use that to put thier account on hold for a period of time, OR delete the account.

It means they've gone away for awhile... you can't contact them until they return.

It means, they're profile has been deactivated. That notice will disappear once they reactivate.

What confuses me, is occasionally I've clicked on a profile, and it takes me to a blank page that says "this member is no longer on similarworlds". So I figure they must have deleted their profile permanently, only to discover they've come back at a later time. I've yet to figure out how to deactivate my profile in that manner, just the former.