Kelly In Oil and Gas Rigs Guide
Oil & gas Kelly is a square or hexagonal pipe that fits into the drilling rig rotary table bushing (Rig components). They turn to the right as the rotary table turns. In other words, Kelly's mission is to transfer energy from the rotary table to the drill string.

We can say that kelly is a direct connection between the rig's surface equipment and the oilfield drilling bit and is a vital element of the rotary system.
https://www.drillingmanual.com/kelly-oil-and-gas-rigs/