Porosity is the proportion of a fired clay body's volume made up of open, interconnected pore space rather than solid, fused material. It is the underlying property that a body's absorption rate actually measures: a more porous body has more open space for water to enter, and so takes on more water by weight in the standard absorption test.
Not all pore space behaves the same way. Open porosity is pore space connected to the surface, letting water or air move in and out of the body; this is what the absorption test picks up. Closed porosity is pore space sealed off inside the fired material, disconnected from the surface, which does not let water in but still affects the body's density and strength. As a clay body vitrifies, open pores are progressively closed off or sealed as glassy material forms between clay particles, which is why a well-vitrified body's open porosity, and so its absorption rate, drops sharply near its maturity point.
A body's porosity affects far more than whether it holds water. Higher porosity generally means lower strength and more vulnerability to frost damage, since water absorbed into open pores expands when it freezes. It also affects glaze fit and glaze absorption into the body's surface during firing. Some porosity is not always undesirable, though: a more open, porous body tolerates uneven heating and cooling better than a dense, fully vitrified one, since the pore structure gives it more room to accommodate stress, which is part of why raku and some hand-building bodies are deliberately formulated to stay more porous rather than firing to full vitrification. See thermal shock resistance for the mechanism behind that trade-off in more detail, and earthenware and stoneware for how the main clay body families differ on this exact trade-off.
Porosity does not necessarily reach zero even at a body's own point of full maturity. A well-vitrified porcelain can reach genuine zero porosity, but denser stonewares and earthenwares, having coarser particles in the body to begin with, typically level off at a small but real minimum instead, as much as 3% in some cases, rather than closing off completely. Firing further in an attempt to eliminate that last porosity risks bloating the body rather than densifying it any further.