The One Piece world is mostly made up of islands. There is one long mass of land called the Red Line that goes all the way around the world. On either side of the Red Line are oceans called North Blue, West Blue, East Blue, and South Blue.
The Grand Line is an ocean that goes all the way around the world; it connects to the red line in two places. On either side of the Grand Line is the calm belt. In the calm belt, there are giant sea monsters and no wind, making it difficult for boats to enter the Grand Line.
Normal compasses don't work inside the Grand Line. You have to use a Log Pose.