To soak is to submerge a food - usually something dried, like beans, grains, or dried mushrooms - in liquid for an extended period so it rehydrates and softens before cooking. Soaking dried beans and legumes overnight shortens their cooking time and, when the soaking water is discarded and replaced, reduces some of the compounds that cause digestive discomfort.
Grains like rice, farro, and other whole grains are sometimes soaked before cooking to soften tough bran layers and shorten cooking time. Soaking is also used to extract flavor rather than just soften texture: dried mushrooms or spices steeped in hot water or broth yield a concentrated liquid used in sauces and soups, and nuts soaked in water soften for blending into nut butters or dairy-free milks.
Soaking is distinct from brining, which uses a salted (and sometimes sweetened) liquid specifically to season and retain moisture in meat, poultry, or seafood, not to rehydrate a dried ingredient.