In the portafilter where the water and coffee come into contact during the brewing process, tamping is used to compress loosely dosed coffee grinds into an equally distributed puck.
Using tamping pressure, espresso ground coffee is packed (or "tamped") into the basket of an espresso machine. To ensure a perfect shot, a tamper is used to uniformly pack the grounds for the shot. A loose pile of "dosed" grounds is converted to an enclosed cake in the basket.
Tampers range in size from 48 millimetres up to 58 millimetres, with the majority of espresso makers making use of one that is the latter. It's not uncommon for tampers to state their basket's size directly in their name! (i.e. The Motta Professional Coffee Tamper, 58mm). They are made from high quality materials such as Australian oak and stainless steel, meaning they make the perfect espresso and are easy to clean.
The portafilter is the right size for your tamping and is made of light, durable metal. By the way, if you're working as a barista and have a lot of baskets to handle, you'll need extra espresso tampers or a convex tamper.