What is D-Mail Steins Gate?

Excuse me, could you please explain what exactly is D-Mail in the context of Steins;Gate? I've heard it mentioned a few times but I'm not entirely clear on its significance or function within the story. Is it a form of communication or technology that plays a crucial role in the plot? I'm quite intrigued and would love to know more about how it fits into the narrative and the world of Steins;Gate.