Tamping
German: Tampern Also known as: Anpressen
Pressing the ground coffee in the portafilter flat with a tamper so that water flows through evenly.
In tamping, the ground coffee in the basket is compacted smooth and level with a tamper. This creates an even coffee bed through which water flows at a similar speed everywhere. If it is uneven, the water looks for paths of least resistance.
Such "channeling" leads to uneven extraction: some areas become over-extracted, others under-extracted. A straight, even pressure matters more than a great deal of force.