Below are some basic architecture diagrams showing external access to an internal Tableau Server with and without the Rapid Portal.
Without Rapid Portal
Without Rapid Portal, external users (customers, partners, vendors, and external employees) must connect directly to Tableau Server inside your company's firewall.
From Tableau documentation:
In most enterprises, Tableau Server needs to communicate with the internet. Tableau Server was designed to operate inside a protected internal network. Do not set up Tableau Server directly on the internet or in a DMZ. Instead, communications between your network and the internet should be mediated using proxy servers. Forward proxy servers mediate traffic from inside the network to targets on the internet. Reverse proxy servers mediate traffic from the internet to targets inside the network.
With Rapid Portal
Rapid Portal gives external users (customers, partners, vendors, and external employees) interactive access to embedded Tableau content without giving them direct access to Tableau Server.
In this use case, Rapid Portal is serving as a reverse proxy for Tableau Server.