it think it is the video when they show the new underworld. 
Terraria Community Forums
Hyixtronix
if they make one? dont they always make one? and DG-VR shouldent it say "Terraria 1.3 gameplay trailer?"
Dathir
They're probably not gonna make one. In the past trailers have announced the release date of the update but the devs have already released a release date video
Sixshaman
Actually, the release date of 1.2 was announced before the 1.2 trailer. Anyway, there are only 10 days left, we just need to wait.
Hyixtronix
okay but mabey there will be one this Tuesday
If they make one.