The windscreen washer bottle is the container used to hold the windscreen washer solution which is used to spray the windscreen to help keep it clean and free from debris. It pumps out a spray, which works with the wipers to clean the windscreen.
The windscreen is fitted with washer jets which are connected to the washer bottle by a pipe or hose which carries the washer solution to the jets. The washer jets are usually located beneath the windscreen on the bonnet. The windscreen washer bottle is located beneath the bonnet and connected to the windscreen washer pump which is powered by the windscreen washer motor. Together they work to provide the power and motion to deliver the spray when it's needed.
The driver can choose to spray the windscreen by using a control usually situated on the windscreen wiper switch stalk which tends to be located on the steering wheel column. The washer solution can contain de-icer which helps to stop the windscreen from freezing over in colder months.
The usual reason for replacement of this part is old age or damage. The bottle is usually made from plastic and will eventually deteriorate, and the connecting tube can also suffer from wear and tear. Replacing the windscreen washer bottle with a windscreen washer bottle will ensure you get a part that's compatible with your vehicle.