What is a opentable reservation?

I'm so new to this world, i really don't know, and I really want to get a really Fancy restaurant not to Fancy, but that looks okay, in Washington DC, so I don't know what is a open-table reservation. I'm trying to impress my girlfriend mother, but i don't know which or how to get a great restaurant, I'm so lost! D: please help a beginner ! :D