This is a pretty broad question, but at a high level Power Apps Portals allow you to quickly build web applications similar to a model-driven app, but expose it to users that don't have a Power Apps or Dynamics license. Most often Portals are meant for external users.
Power Apps Portals are meant for interacting with data stored in the Common Data Service (CDS), including Dynamics 365 Online. They don't leverage the connector framework that Power Automate or canvas-based Power Apps.
There is a learning curve, but once you know how to use them, you can build web applications quickly. You can take customize them even further using JavaScript.
Feel free to post any specific questions and I'll do my best to help.