A normal SIM card is the tiny plastic chip your phone company gives you to get you online and make calls. An eSIM does the same job, but there is no plastic chip. It is a SIM that lives inside your phone as software. You add it by scanning a code, not by opening a tray with a pin.
For a trip to Puerto Rico this is great news. You do not have to find a shop in Old San Juan or wait in a line at the airport. You buy your data online, scan a code, and your phone is ready. When you land at Luis Munoz Marin Airport, you walk out connected and can open your maps right away.