Proof of onward travel is a document showing you have a confirmed plan to leave a country. Immigration officers at 50+ countries require it — and you can get it free in 30 seconds.
Proof of onward travel is any document that shows you have a flight, bus, train, or ferry ticket leaving the country you are entering. It proves to immigration officers that you do not intend to overstay.
A dummy ticket (also called a flight reservation or flight itinerary) is the most common and accepted form. It looks exactly like a real booking confirmation with a PNR code and QR code.