A 
top hat washer is a type of plumbing spacer and seal, typically made of plastic, used to secure taps and prevent damage when fitting them to sinks, basins, or baths. Its shape resembles a top hat, with a wide, flat brim and a raised, hollow core. 
Primary Purpose and Uses
Prevents damage: The wide "brim" of the washer creates a larger surface area, preventing the tap's backnut from cutting into the material of the sink, especially for thinner or softer materials like fibreglass or composite basins and baths.
Provides stability: It helps hold the tap securely in place, stopping it from becoming loose during use.
Fills gaps: It can be used as a spacer to fill a gap if the tap's thread is not long enough to be securely tightened against the mounting surface with a standard washer.
Universal fit: Many modern top hat washers are designed to be universal, fitting both 1/2" and 3/4" BSP (British Standard Pipe) threads, making them versatile for various tap styles like monobloc, single lever, pillar, and bridge taps.