Yes, it is flux (solid rosin). He vaporizes some with the iron and the smoke resolidifies on the contacts, so there is a very thin layer of flux to help with the soldering.
When you touch the iron with the rosin, it starts smoking like incense at a church (same stuff actually), but a small amount remains on the tip. You transfer this to the contact you’re soldering, dip into the rosin again, and then solder as usual.