Feature Flag and its use in Software Testing

Hi this is Palak Sharma
I am in final year of my engineering. We are planning to develop a software application for final year project, campus interview is also near and this software application will also help in that phase. for testing phase I am learning about Feature Flag and other things, I am little bit confuse about how feature flag can be use and when it should be used in testing phase, can anyone give their suggestions on this, would be a great support in this regards.
Hoping for some positive response.
Thanks