Planetary gears

Summary
A simple planetary gearset has a sun gear, planet pinions mounted on a planet carrier, and a ring gear.

Most automatic transmissions use epicyclic or planetary gears. They are constantly in mesh with each other.
A basic planetary gearset has a sun gear, which meshes with planet gears, also called planet pinions.
The planet pinions, in sets of three or more, rotate on bearings, on hardened steel pins, on a planet carrier, which spaces the pinions equally around the sun gear. It also locates them so they can mesh with an internally toothed ring gear.
This means the planet pinions are always in mesh with the sun gear and the ring gear.
In operation, their motion is described as either “Walking" or "Idling".
“Walking” means that if either the sun gear, or the ring gear, is held stationary, the alternative driving member rotates the planet gears on their pins. This turns the planet carrier in the same direction as the driving member.
Planet gears always turn in the same direction on their pins as planet carrier rotation, while they walk around a stationary sun gear. They always turn in the opposite direction on their pins while walking inside a stationary ring gear.
"Idling" refers to the rotation of the planet gears on their pins whenever the planet carrier is stationary. Torque is transmitted from the sun gear to the ring gear, or from the ring gear to the sun gear, via the planet gears and the stationary carrier.
In both cases the driven member is turned in the opposite direction to the driving member. To provide the ratios available from the gearset, one or more of the components must be held - or released. This is normally done by hydraulic servos, operated by transmission fluid under pressure, acting on lined bands or clutches, or, by one-way clutches. They allow turning in one direction, but act as a lock-up, or reaction member in the opposite direction.
In practice, a combination of these is normally used.