I wondered this before myself, I found that wikipedia had most of my answers.
The Forgotten Realms is a fantasy world setting, described as a world of strange lands, dangerous creatures, and mighty deities, where magic and supernatural phenomena are quite real. The premise is that, long ago, the Earth and the world of the Forgotten Realms were more closely connected. As time passed, the inhabitants of planet Earth have mostly forgotten about the existence of that other world – hence the name Forgotten Realms.
Check it out: https://en.wikipedia.org/wiki/Forgotten_Realms