Permeability
A rock's ability to transmit fluids through its connected pore spaces; together with porosity it governs how readily a well produces oil or gas.
Permeability is the measure of how easily fluids — oil, gas, or water — can move through the connected pore spaces in a rock. It's usually expressed in units called millidarcies. Permeability is distinct from porosity: porosity is how much open space a rock has, while permeability is whether those spaces connect well enough to let fluids flow.
The two together govern how a well produces. A rock can be full of oil (good porosity) but barely give it up if the pores aren't connected (poor permeability). Conventional reservoirs like sandstones have relatively high permeability, so oil flows to the wellbore on its own.
Shale and other source rocks have extremely low permeability — which is exactly why a shale play needs hydraulic fracturing to create artificial pathways. For owners, low natural permeability isn't a dead end; it's the reason modern completion technology, not just the rock, determines whether your minerals produce.