a 16-bit unsigned number that uniquely identifies a Network application or service on a host. Port addresses are included in a TCP or UDP header to associate a Payload with a specific process or service. Using a distinct port for each process or service allows a single device to simultaneously recognize multiple traffic types. Ex: A device can simultaneously receive website traffic and email traffic because each service is associated with a unique port.
Source Port
Source ports are generally chosen by the IP stack to be unique among all open conenctions (usually Ephemeral)
So each open connection has a unique ordered quad, allowing connections to be kept separate even though the IPs, and destination port are the same.
FYI each host here is keeping track of 4 sequence numbers. See TCP
Well-Known
Privileged/Reserved 0-1023 reserved for commonly used services
Registered
1024-49151 registered with IANA for specific use
Ephemeral
Private/Higher Number/Dynamic 49152+ dynamically assigned