What is Rocket?

I keep reading about “Rocket”. Can anybody tell me what it is?

An old project when UE4 was being tested in the past.

Rocket was an internal code name for the release of UE4 to everyone. There are still some references to it in the code base which mostly refer to when you are building your game library but not for building the full engine. But there shouldn’t rely be any reason you need to concern yourself with those aspects.