An endpoint is a host that acts as the source or destination of data traffic flowing through a network.
When you are at your PC, editing your CV and uploading it to a file server, you are sitting at an endpoint.

In Cisco wireless terminology, bridge mode is used when an AP acts as a dedicated point-to-point or point-to-multipoint bridge between networks, essentially serving as the primary hub that connects multiple remote units (clients or secondary bridges) in a point-to-multipoint setup.

The root bridge is determined by the lowest bridge ID, which consists of the priority value and the MAC address. Because the priority values of all of the switches are not avalable, the MAC address is used to determine the root bridge. Because S3 has the lowest MAC address, S3 becomes the root bridge.