It is designed to launch payloads into space.
The majority of the fuel of a conventional rocket is used to get the rocket from the ground up to around 20,000 feet, where the first stage is then jettisoned and the second (and third, if equipped) stage puts the payload into orbit.
This plane is designed to carry a rocket up to 20,000 feet then launch it, in essence becoming the 'first stage.' Being an airplane, it can then return to base and be used over and over again, saving money.
Here is an artist's drawing of what it would look like:
