How much water moves underground and how fast it moves depends on a number of factors. Two important factors are the porosity and permeability of the rocks.
For groundwater to be able to get into a rock with good porosity it must also have good permeability. For a rock to be permeable and for water to move through it, the pore spaces between the grains in the rock must be connected. Permeability is therefore a measure of the ability of water to move through a rock.