system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is the impact of browsers no longer accepting 3rd party cookies and Apple’s Intelligent Tracking Prevention?

I know it impacts advertisers ability to target, but would love a clearer explanation of how it works and the impact.
gpt
An analogy:

Every time you go anywhere in public, the shops you use, the buildings you enter, and the people you interact with put a coloured sticker with a number on you.  It's just a sticker, it contains only a number, it's not "private".  Say the baker always uses a green sticker, and he numbers based on the order you walked into his shop.  And the butcher uses a purple sticker and he numbers based on a random number that he makes up.  And the grocer uses a green sticker and he numbers based on how much you buy from him.

Whatever.  It doesn't matter.  The butcher, the baker and grocer don't know what the other people's numbers mean, it's just a number.

And when you get home, your arm is full of coloured stickers with numbers on.  But it means that when you go out tomorrow, the butcher knows that you're #27, that you buy beef from him regularly and that yesterday you were interested in how to best cook steak.

Not a problem.  The grocer knows nothing about what the butcher's number means or what the butcher knows about you.

The problem comes when the butcher, the baker and the grocer all employ a company to put those stickers on you, because they don't want to do it themselves.  The company does it "for free" to them, and labels you with a pink sticker with a unique number.  When the butcher asks and says that you have a pink number #35 on you, the company can tell him everything he'd normally store about you (because the company have recorded it for him).  When you go to the grocer, he can also talk to the same company and ask them for everything he wanted to remember about pink #35.  Still not a problem.

But now that one company runs all the data collection for lots of people.  So they can tell the butcher that you went to a rival butcher's last week because your pink #35 was spotted there.  The butcher can ask for other information about pink #35, so he knows that you bought turkey gravy yesterday and maybe he can try to sell you a turkey today.

And the company then sells that data about pink #35 to completely unrelated companies that you've never dealt with, say a clothing store, so they can suggest that if you're eating that much meat, maybe you should try a bigger size of jeans, and so on.

The stickers are cookies.  The company are data aggregators like Google ads, many tracking cookie and analytics firms, and the average website has something like 35 companies that put stickers on you where those stickers are shared with EVERYWHERE you go which uses that same company.

Apple's (not new, unique or innovative) idea is to keep your arms covered so you only show the stickers you want to the companies that need them and when you go to the butchers they have to give you a new sticker from the company because you refuse to show them your previous ones, so they have no idea who you are.  So they can't tie in that information about you from across the net, sell it, and use it in potentially nefarious ways.

And occasionally, they'll take the stickers off you entirely because you haven't needed them in a while.