The Start Area is the first part of a board that begins the game. Every Mario Party game has had some kind of Start Area, and later entries have featured variations on the Start Area, such as the Bowser Start Area in Mario Party 4. The Start Area is where basic functions of the game take place, such as explaining the rules of the board are displayed and deciding the turn order.
There are a few circumstances where in-game events take place in the Start Area. Sometimes on a certain board if you land on a Happening Space, you will be sent back to the Start. In Mario Party 1-8, the Last 5 Turns event is held in the Start Area, where the board host will at times direct players to perform certain tasks. This, however, does not affect any of the player positions.