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In an oversimplified explanation of drilling a hole with a drilling rig,
the operation can be compared to digging a hole with a shovel.

When you dig a hole with a shovel, you throw the dirt out of the hole in order to make the hole deeper.

In its simplest form, this is what happens when you drill a hole with a drilling rig.

A drill bit is put on a piece of drill pipe and the power of the rig
rotates the pipe and bit digging a hole form the surface of the ground
to the depths of up to several miles deep. This drilling operation
creates the dirt, called cuttings that must be removed from a hole.

Drilling fluid is pumped down the center of the drill pipe. When the fluid gets
to the drill bit it is released into the hole. The fluid then forces
the cuttings that are created by the drill bit, back up the hole to the
surface. It is at this point that the rotating head comes into play.

The first purpose of the rotating head is to control the direction of the
drilling fluid and cuttings as they return to the surface. This is
accomplished because the rotating head makes a rotating seal around the
drill pipe and it also makes a seal between the hole and the surface.
These seals force the fluid and cuttings away from the hole and back
into the drilling fluid system where the cuttings and hydrocarbons can
be separated out.

Another important purpose of the rotating head is to help control formation
pressures created by the hydrocarbons. These pressures can come in the
form of a drilling break and resulting "kick" as the hydrocarbons come
to the surface unexpectedly. They may also come from purposely drilling
the well under balanced in order to protect delicate formations.

The most important purpose of the rotating head is to help insure the safety of rig personnel, equipment and the environment.

The rotating head is, therefore, a rotating sealing device for circulating
and retaining drilling fluids and restraining hydrocarbons. Its use can
be effective during the entire drilling operation.