...
The purpose of spanning tree protocol is to provide the ability to create loop-free Layer 2 topologies while having redundant links. While connecting multiple bridges or just cross-connecting bridge ports, it's possible to create network loops that can severely impact the stability of the network. Spanning tree protocol aims to resolve this problem by introducing the concept of the root bridge, all bridges in the same Layer 2 domain will exchange information about the shortest path to the root bridge. Afterward, each bridge will negotiate which ports to use to reach the root bridge. This information exchange is done with the help of Bridge Protocol Data Units (BPDUs). STP will disable certain ports for each bridge in order to avoid loops, while still ensuring that all bridges can communicate with each other. For an in-depth description of protocol please refer to IEEE 802.1Q1D.
As a best practice, it is always recommended to manually set up each bridge's priority, port priority, and port path cost to ensure proper Layer2 functionality at all times. Leaving STP related values to defaults are acceptable for a network that consists of 1 to 2 bridges running with (R/M)STP enabled, but it is highly recommended to manually set these values for larger networks. Since STP elects a root bridge and root ports by checking STP related values from bridges over the network, then leaving STP settings to automatic may elect an undesired root bridge and root ports and in case of a hardware failure can result in an inaccessible network.
...