ESC/P (Epson Standard Code for Printers) is a language used by printers like Epson to understand what you want to print. It is kind of like if you were speaking to the printer and needed to make sure it understood what you were saying, so you used a special language that only the printer could understand.
Using the ESC/P language, you can tell a printer things like what kind of font to use, how big to make the text, where to put the text on the page, and even how to draw pictures. It's like giving the printer a set of special instructions so it knows exactly what you want it to do.
Because different printers understand different languages, having a standard language like ESC/P makes it easier for different computers and devices to talk to Epson printers no matter what brand they are, sort of like how people can speak different languages, but still understand each other using a common one.