It's the carbonate-silicate cycle. Erosion of silicate rocks (which form most of the Earth's crust) by chemical weathering consumes CO2 from the atmosphere and turns it into thermodynamically stable carbonates. Those carbonates can eventually be subducted and their CO2 released again by heat from the Earth's interior.
https://personal.ems.psu.edu/~jfk4/PersonalPage/ResInt2.htm
The carbonate-silicate cycle, which plays a key role in stabilizing Earth's climate over long time scales, is shown in Fig. 2. The cycle begins when atmospheric CO2 dissolves in rainwater, forming carbonic acid, H2CO3. Through a process termed "weathering", this weak acid dissolves silicate rocks on the continents, releasing Ca++, Mg++, HCO3- (bicarbonate), and SiO2 (dissolved silica) into solution. The products of weathering make their way down to the oceans in streams and rivers. There, organisms such as the planktonic foraminifera that live in the surface ocean use them to make shells out of calcium carbonate (CaCO3). When the organisms die, they fall down into the deep ocean, where most of the shells redissolve. Some of the calcium carbonate survives, however, and is buried in sediments on the seafloor. The seafloor spreads from the midocean ridges and, at some plate margins, is carried down subduction zones. The carbonate minerals recombine with SiO2, which by this time is the mineral quartz, to reform calcium and magnesium silicates and release gaseous CO2. This CO2 is vented into the atmosphere through volcanoes, thereby completing the cycle.