An eSIM is a digital version of the physical SIM card that allows you to activate a cellular plan from your carrier without having to use a physical SIM. It is programmable remotely via software and is built into newer smartphones. An eSIM works the same way as a traditional SIM card but you don’t need a physical one to use it.
The Pixel 4a does NOT offer an eSIM.
The Pixel 5 and 6 DO offer an eSIM.
In our experience, the eSIM tends to be more prone to issues and errors when activating and setup, however, once you can actually get it working it serves the purpose as outlined above. We have not had experience with Visible although I would think their customer support team can help walk you through the setup if you have a Pixel 5 or 6 or other eSIM compatible device.