Port <number> is part of switch internal port numeration on device that is needed to establish STP port identifier. Physical port numeration does not need to match exact physical port number (and sometimes even can't match since device can have more than one line card). Typically, physical port count starts with 1 and each next port increases by 1, but it may depend on hardware model and line card that is in use.
In the case above port eth0/0 is port 1 and eth0/1 is port 2. If you want to check each port number on device you can do it by issuing command show spanning-tree detail. Port number in example above is showing which port is chosen to be STP root port on local device.
Port priority (default value is 128) and port number together make port identifier (the same way as STP priority and MAC address create STP bridge ID). Port identifier is last option that can/will be used in determination of port which will be unblocked, if all other previous steps in STP election fail to determine best path to root bridge (example would be 2 switches connected to each other with two or more links - port identifier is option of last resort that can be used to determine which port should be elected as root port (will be in forwarding state) on device which lost STP root bridge election).
https://www.cisco.com/c/m/en_us/techdoc/dc/reference/cli/nxos/commands/l2/show-spanning-tree.html