Definition - What does Drill String mean?
A drill string is a column, or string, of drill pipe that transmits drilling fluid (via the mud pumps) and torque to the drill bit. A drill string is made up of consecutive drilling rods attached to the drilling head on one end and the drilling rig on the other. As the drilling head progresses through the bore, drilling rods are added to the string to advance the drill bit and pull back spoil. The drilling string connects the driving equipment to the bit and provides the force for advancing the bit.
Trenchlesspedia explains Drill String
The drill string transfers the driving force from the rig to the drill bit through rotational energy and thrust. The rods are also hollow which enables the supply of drilling fluid to keep the components cool and to transport the cuttings back to the surface. Some level of directional control is possible by stopping the rotational motion and pushing on the string where the drill bit will then shift it's alignment whereby the pressure of the soil on the slanted edge will cause a change in direction. Cables are also routed along the drilling string which communicate with sensors.