Grog is clay that has already been fired, then crushed or ground into granules and added back into an unfired clay body as a non-plastic filler. Because it has already been through a kiln once, grog itself no longer shrinks or reacts chemically the way raw clay does, which is the source of most of what it contributes to a body.
Grog opens up a clay body's structure: the granules interrupt the tightly packed plate-like structure of raw clay particles, giving trapped water and gases more paths to escape during drying and the early stages of firing. That same open structure is what raises a body's thermal shock resistance, since the granules interrupt crack propagation through the body and give it more room to accommodate uneven expansion and contraction without failing outright. A grogged body also shrinks less overall, since the grog itself contributes bulk without shrinking, which reduces warping and cracking risk on large or slab-built work in particular.
Grog reduces a body's plasticity, since the hard, inert granules interrupt the plate-like clay particle structure that makes a body workable on the wheel. It also roughens the surface, sometimes visibly so, depending on particle size. Grog is sold and specified by mesh size (a measure of particle size, where a higher mesh number means finer particles): fine grades around 48 to 80 mesh are barely noticeable in a finished surface, while coarse grades around 20 to 30 mesh give a pronounced, gritty texture, more common in hand-building and sculptural bodies than in throwing bodies, where a smoother feel under the hands is usually preferred.
Most grog is simply pre-fired clay of whatever color it started as, which can leave visible dark specks in a light body, since ordinary fired clay carries iron and other impurities. Molochite, a specific commercial product made by calcining low-iron kaolin rather than firing ordinary clay, is the common workaround where speckling isn't wanted: it opens up a body's structure the same way any grog does, but stays nearly white, which is why it shows up specifically in porcelain and other light-colored bodies where a fired-clay grog's specks would be a problem.
How much grog goes into a body depends heavily on what the body is for. A throwing body aimed at smooth functional ware might carry very little grog, or none, to keep the surface pleasant under wet hands. Sculptural and hand-building bodies commonly run higher, often in the range of 10 to 25% grog by weight, since the durability and thermal shock benefits matter more there than surface smoothness. Bodies built for large-scale or architectural work, or for techniques with severe thermal shock like raku, sometimes run considerably higher still, trading most of their plasticity away entirely for structural stability at scale.