Imagine that you are pulling a train along it's track. You make it move so you do work on it.
Now imagine that another guy is pulling sideways. Perpendicular to the tracks. Of course the train doesn't move that way. His effort does not cause any displacement. So he wastes his effort and does no work.
In mathematical terms, we say that the force $\vec F$ and displacement $\Delta \vec x$ must be parallel:
$$W=\vec F \cdot \Delta \vec x$$
The dot product is zero when vectors are perpendicular.
If the displacement is perpendicular to the force you apply, then it is not you who is the cause of that displacement. So it is not you who does any work.
A satellite in a perfectly circular orbit is moving tangentially to the earth's surface - parallel to the ground. Gravity is pulling straight downwards, perpendicular to that orbital path. So gravity does no work.
Gravity does cause the satellite path to turn, so that it turns slightly in the next instant - but in that next instant, the force of gravity has turned slightly as well and is again perpendicular. So even though gravity causes turning (which causes the orbit to be circular), it causes no displacement. Gravity does no work on that satellite in a perfectly circular orbit.