A strut mount is like a sandwich. One side bolts to the vehicle, the other side to the strut. In the middle is a rubber-like insulating material. So as the vehicle moves and goes over bumps, the up & down impact pushes and pulls at the mount. The mount's job is to cushion the impacts to reduce the jarring effect, noise and vibration that could transmit into the vehicle.
On many front struts, the mount also includes a bearing that the strut attaches to. With one on each side of the vehicle, these bearings act as the steering pivots. The bearing is a critical component that affects steering movement smoothness and response.