Ansible: What are the network bridges for?

Hello, I have trouble understanding the section about [network bridges in this deployment guide](https://docs.openstack.org/project-deploy-guide/openstack-ansible/rocky/targethosts.html#configuring-the-network). These are the general questions I'm trying to find answers for: - What are the bridges necessary for? How are they used? (Why) Isn't it possible to use the physical interfaces directly? - What are the bridges connecting? - (How do I configure the bridges for my setup?) My setup: I have two identical machines running Ubuntu 18.04, both have two network interfaces. I know I'm asking really basic questions here, but that's because I feel the docs are seriously lacking here, and the basic info I read about bridges didn't help me to understand what I'm supposed to do at this point of the deployment. Thank you to everyone who's trying to help me!