The question is rather like "What is grief?" or "What is conversation?" or "What is joy?" A simple question but with an answer that could fill and has filled whole books. Yet, it is also one of those questions that is simply unanswerable without experience. And to experience grief, conversation or joy is to know what it is. So it goes with prayer, it can be hard to define neatly what it is - and to be truthful it will take different forms at different times and in different settings and through different traditions - yet when you have an experience of prayer and know that something about you has changed as a result, you then know what it is about.
What is prayer? Try it to find out for yourself.