A shower hose washer is a crucial, inexpensive flat rubber or silicone gasket that fits inside the ends of a standard shower hose to create a watertight, leak-proof seal at the connection points. 
Key Details
Material: They are primarily made from durable rubber (often EPDM) or silicone, materials chosen for their flexibility and resistance to hot water and wear.
Size: Almost all standard household shower hoses and shower mixers use a 1/2" BSP(British Standard Pipe) thread. The corresponding washers typically have an external diameter of around 19mm (0.75 inches), an internal diameter of about 11mm, and a thickness of 2mm or 3mm.
Function: The washer provides a tight seal between the metal or plastic nuts on the hose ends and the corresponding shower outlet or hand shower unit. Replacing a worn-out washer is a simple and effective way to fix a leaky shower connection without replacing the entire hose.