referer spoofing is like pretending to be someone else. Imagine your friend saying they got you a gift, but actually it was someone else who gave it to you. That person is pretending to be your friend.
In the same way, when you click on a link to go to a website, your computer tells the website where you came from. This is called the "referer." But sometimes people can pretend to be someone else and make the website think you came from a different place.
This can be a bad thing, because it can trick the website into thinking you're someone else entirely. For example, someone could use referer spoofing to make it look like a really popular website (like Google) sent them to a different website. The different website would then think that lots of people are coming from Google, even though they're not really.
Overall, referer spoofing is like cheating at a game by pretending to be someone else. It can be harmful and can trick people into thinking things that aren't true.