A hot solder iron tip will oxidize. The tip is made out of copper (and maybe nickel and other metals) and there's a tin plating on top to protect the copper from oxiding.
When tip stays in the air, the tin is oxidized, dirty and so on. When you start the iron after a long pause, you want to remove the top layer of tin that got oxidized because you don't want to mix the solder with the impurities on the tip. However, if you keep removing micro-layers of tin from the plating, eventually you'd get to the copper and damage the tip.
So it's a good idea to add some "sacrificial" solder to the tip before you turn it off. That extra solder will protect the tip and its tin plating and can be removed when you start working again with the iron.
Therefore .. start iron , wait to heat up, clean solder left on tip before you turned it off previously, add a bit of fresh solder so that the tip won't oxidize as you keep the iron idle and also to increase surface area between the iron tip and whatever you want to heat up.
Before you turn it off, add some solder to the tip so that you'll have something to remove when you start working again.
Watch this as well:
and this from about 1:00 (a bit exagerrated, but he solders according to some stricter standards) :
and this from about 6:20 (if you don't want to watch it all) :
Pay attention to how he works with the soldering iron.