The shipper is the origin of a load — the company or person that owns the goods (or is responsible for them) and sets up their transport. They’re the customer side of a haul, the one that needs freight moved.
The shipper hands off to the carrier that hauls the load and to the consignee that receives it, with the bill of lading documenting the whole arrangement. In many cases the shipper is also the consignor, the party the goods are sent from.