A 15mm x 15mm x 3/4" washing machine tee is a plumbing fitting used to branch a standard 15mm water supply pipe to provide a connection point for an appliance with a 3/4" BSP hose connection, while allowing the 15mm pipe to continue. 
This type of fitting is commonly referred to as a washing machine valve tee or an appliance tee tap. 
Key Features and Specifications
Function: Allows a washing machine, dishwasher, or other appliance to be connected to a standard 15mm water supply line, while also maintaining the flow through the main 15mm pipe run.
Connections:
Two 15mm compression ends (one inlet, one outlet on the straight run) for connecting to 15mm copper or plastic pipe (olives are usually included).
One 3/4" BSP male threaded branch for screwing directly into a standard appliance fill hose.
Material: Typically made from durable, chrome-plated brass to resist corrosion.
Operation: Many versions include a built-in isolation valve (usually a quarter-turn lever/handle) at the 3/4" outlet to control the water flow to the appliance independently.
Handles:  Supplied with interchangeable red and blue handles to denote hot and cold water feeds.
Approvals: WRAS (Water Regulations Advisory Scheme) approved products are widely available, ensuring they are safe for use in UK water systems.