TCP port 1723 is used by PPTP, a legacy VPN protocol, and it shows that a port can represent only one piece of a larger tunnel.
A port is a transport layer number used together with an IP address and a protocol such as TCP or UDP to direct traffic to the correct service on a host.
A server process binds a socket to a port and listens, while a client typically chooses an ephemeral source port for outbound connections.
The combination of source and destination IP addresses, source and destination ports, and the transport protocol uniquely identifies a flow so the operating system can keep many conversations separate.
Firewalls, NAT, and scanners talk about ports because the destination port is the stable rendezvous point that exposes a service to the network.
PPTP uses TCP 1723 for the control channel where the client and server negotiate tunnel setup and authentication.
The actual user data is carried separately using GRE, which is IP protocol 47, not TCP or UDP, so a working PPTP connection typically requires both TCP 1723 and GRE to pass through firewalls and NAT.
In practice, a client first establishes the control connection to port 1723, the tunnel is created, and then PPP frames are encapsulated inside GRE for the data path.
Security wise, PPTP is considered obsolete due to known cryptographic weaknesses, so seeing 1723 open often indicates legacy remote access that should be migrated to modern VPNs.