Refers to a fastening or binding.
La amarradura de los paquetes debe ser lo suficientemente fuerte para soportar el peso durante el transporte.
The binding of the packages must be strong enough to support the weight during transport.
La amarradura de la carga era tan fuerte que no se movió en todo el viaje.
The binding of the cargo was so strong that it didn't move at all during the trip.