It marks the pickup location whose address is used as the ship-from address on Shippo rate requests and labels. Only one location can be the sender: enabling it on one location disables it on all others (the form notes this). The location must be active and have address, city, state and zip filled in, otherwise checkout fails with Shippo: Your Primary Shipping Sender Location is missing required address fields…. If no sender is set the Pickup Locations page shows a yellow Shippo Integration warning and customers see Shippo: Please set a Primary Shipping Sender Location in your Pickup Points settings.