A Tap washer is a thin disk with a hole in the middle that is designed to even out an uneven surface so that the fastener can be securely attached to the surface. Tap washers are also known as sealing washers these type of washers are designed to prevent leakage and contamination. Tap washers come in a range of sizes and in various materials.
When a tap is turned on the washer is directed towards the seat of the tap. This creates a build-up of pressure, forcing the water out of the tap. When the tap is turned off, the washer creates a watertight seal by pushing against the seat which stops the flow of water.
If your tap is dripping , no matter how much you tighten the tap then there is a good chance that the tap washer is worn or damaged and needs replacing. Tap washers are prone to wear and tear and when they are worn it allows a small amount of water through which drips from the tap.
It will depend on the size of your tap, most kitchen and basin will have the same tap size, however bath taps may be larger. You can always take the old tap washer with you to the DIY store if you are unsure of the size you need to replace.