A normal SIM card is the tiny plastic chip you push into your phone. It tells your phone which network to use and how to get online. An eSIM does the exact same job, but there is no plastic chip. It is software that lives inside your phone already.
To use it, you scan a QR code or tap a button. Your phone then downloads the eSIM and connects to a local network. Nothing is shipped to you. Nothing needs to be swapped out. You can set the whole thing up while you are still at home before your flight to Qatar.