The rock cycle is the process by which rocks of one kind change into rocks of another kind.
There are three main kinds of rocks: igneous rock, metamorphic rock, and sedimentary rock. Each of these rocks can change into the other kinds by physical processes: cooling, melting, heat, weathering/erosion, compacting (squeezing tightly together), cementing, and pressure.
Sediment is bits of rock that have been broken off of other rock by erosion.
Igneous rock is hardened magma (liquid rock underground). It can melt into magma, erode into sediment, or be pressed tightly together to become metamorphic.
Metamorphic rock is igneous or sedimentary rock that has been heated and squeezed. It can erode into sediment or melt into magma. It formed under extreme pressure and temperature deep inside mountain chains.
Sedimentary rock is compacted sediment which can come from any of the other rocks, plus remains of living things. It can erode back into sediment, or be pressurized into metamorphic rock.
These processes can occur in different orders, and the cycle goes on forever. The eroding is happening all the time by water and wind.