its an sql injection attack.
its rather unlikely that it works in a modern app.

assuming this would work,
it injects a command in the sql database.

it is assumed that the app runs a sql querry with the input field as a parameter e.g.
INSERT INTO "bills" (item, ammount, tip) VALUES ("steak", "20,00 $", "content of the custom tip goes here");

the semicolon indicates the end of the querry,
so the the text would cause the app to run an unfinished querry, and then start a new querry that messes up the content of the bills table.

Nothing. For one, it won't let you enter letters. Two, the table structure to these POS systems are more nuanced than a simple bills table with am amounts field.

It's amusing and all, but it's not something you can do.

Source: work with, and develop around, these types of POS systems.