Understand that the Spanning Tree Protocol controls the switched network topology. Redundancy is essential in modern networks, and without STP, switches would often have multiple paths to get to a given destination. Frame duplication due to the multiple paths, plus nonstop broadcast forwarding, would lead to broadcast storms and general instability.
Understand the importance of the root bridge. The root bridge is the center of the spanning tree universe; all STP calculations are based on which device is the root. You need to know how the root is selected and how to influence the process. Switches calculate which is the shortest path to the root and disable ports that promote redundancy.
Know the different types of ports. The root port is the port on a switch that has the least-cost path to the root bridge. A designated port is a port that is active but does not lead to the root. All the ports on the root bridge are active and are designated ports. You need to know how switches decide what state their ports will be in.
Understand the method of breaking ties. Whenever there is a tie, there is always a method of breaking it. Remember that a lower number is usually better. The bridge ID is a combination of the configured priority and the MAC address, so if two switches have the same priority value, the lowest MAC address will break the tie. If two ports on a single switch can reach the root with paths of the same cost, then the lowest-numbered one is used.