What absorption means
Absorption, in cooking, usually refers to the absorption method, a way of cooking rice and other grains in a measured amount of liquid that is completely soaked up by the time they are done. Unlike boiling in a large pan of water and draining, nothing is poured away. The rice or grain is put in a pan with water or stock, brought to the boil, then covered tightly and cooked on a very low heat. As the liquid is absorbed, the grains swell and soften, and the steam trapped under the lid finishes the cooking.
A short rest off the heat, still covered, lets the moisture even out before the grains are fluffed with a fork. Because nothing is drained, any flavour in the cooking liquid stays in the rice, which is why the method is used for pilafs and rice cooked in stock. It is also how rice cookers work. The amount of liquid depends on the type of grain, so following the packet or recipe is important.
Couscous, bulgur, quinoa and many other grains are also cooked this way. The word absorption is also used more generally, for example when describing how much liquid flour or a dough will take up.