Its called serious 3nt.

 

It applies when you are in a game forcing auction and have found a major fit...

 

eg 1h-2d-2h-3h

 

If you play 'serious 3nt' then 3nt in these auction is never to play. 3nt asks partner to cue bid 1/2 outside controls up the line and indicates 'serious slam interest'.

 

Playing this convention without serious slam interest you still bid 1/2 controls up the line but if you bypass 3nt it says 'I have a 1/2 control in this suit but I don't have serious slam interest'