Definition:
The phase gate is a project management technique that reviews the end of the phase of the project. It is important to create such review to make important decisions to continue on to the succeeding phase, end the project or continue but implement some modifications.
The project is divided into different stages or phases and separated by gates. The process continues at each gate but the project manager or a steering committee should decide whether to continue the project or not.
Further Reading:
Book: The Lean Startup by Eric Ries.