It is a legendary ocean where The One Piece is said to be hidden away. This ocean is said to be the most dangerous place in the world, so dangerous that it's sometimes referred to as the Pirates' Graveyard.
The ocean, though large, is full of pirates from all across the world. Many of these pirates are said to be the strongest in the entire world, and are so dangerous that even the world's governments do not interfere with them. To make matters worse the weather in the grand line is harsh and unpredictable, and normal compasses do not work.