An eSIM is a SIM card that lives inside your phone as software. There is no tiny plastic chip to pop out and no tray to fiddle with. You buy a data plan online, your phone downloads it, and you are connected.
Think of it like buying a movie online instead of a DVD. The data plan arrives by email and installs in a minute or two. Your normal SIM or number stays in the phone, so the eSIM just sits alongside it as a second line for internet only.
For a trip to Paraguay, this means you can land at Silvio Pettirossi International Airport in Asunción and have working internet right away, without hunting for a shop or handing over your passport at a kiosk.