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.

Diagram of Poor and Good Permeability
In the top diagram, even though there is good porosity, there is material (or cement) in the rock to fill in the gaps blocking the pore space between the grains so the water at the bottom cannot flow to the top - the pores are not connected. In the diagram on the bottom, there is nothing to stop the flow of water - the pores are all connected.