Speaking from an American Perspective.
Cultural appropriation is the borrowing, taking, adopting, co-opting of cultural items, tokens, dress, food, words etc. from a disenfranchised culture by the dominant culture.
In America, POC feel a societal pressure to leave the culture and traditions of their ethnic background behind. They're often teased about their clothes, their food, even their parents as children. They are other'd. Not everyone is strong enough to fight, most conform. And then they see the very people who made fun of them eating the food, wearing the clothes and repeating the words. It's suddenly cool and chic for them but just for them. And fast enough to give you whiplash POC are suddenly not ethnic enough. Why don't you dress like this, how come you can't cook like this, what do you mean you can't speak the language. Except even if you can do all those things, you can't win. You're just what they say you're supposed to be.
In a world where racism isn't a thing, cultural appropriation wouldn't exist. We don't live in that world though. Instead we live in a world where Black hair is seen as de-facto unprofessional, where "Where are you from?" is a potentially loaded question. My parents gave my brothers and me the most common American names they could think of to help us assimilate and studies about names on job applications prove their worries correct. You don't reach the finish line by pretending you're already there. Problems don't go away by pretending they don't exist.
The common rebuttals for claims of cultural appropriation are something along the lines of "It's not appropriation, it's appreciation." or "This person from X country disagrees with the whole idea!" or even "You can't just claim only X race can do X, that's racist!". Let's review.
"It's not appropriation, it's appreciation."
This is one that is in fact confused very often because the line in the sand doesn't really exist. Like many things concerning race it's elusive and hard to pin down. My own working definition is based around if the appreciation that is being done is coming from a place of privilege. Because of this the intentions of the person themselves may not matter. Students of law may be offended by accusing people of things they did not have the intent to do but by the same token, ignorance of the law is not an excuse for breaking it. Ignorance of the situation at hand doesn't mean you aren't hurting others.
The most recent viral example I can think of is a few years old. A white girl wearing a Chinese style dress to prom. Oh it took the internet by storm. Boiled down, I called it the difference between being called Chic versus Chink. She liked the dress, it looked good on her, by all accounts she is an alright human being. None of that changes the fact that if a Chinese or Chinese-American girl wore that same dress, looking just as good in it, they run the risk of being called Chink where the white girl was just Chic. Beyond that it also further others the POC girl, a middle finger to the expected assimilation. Cue the expectations that she is multi-lingual, that her english is accented, a second-language if spoken at all. The expectation that she is an outsider grows. A pretty girl in a pretty dress; race doesn't have to come into it but it does and to ignore that is a disservice.
If anyone is waiting for me to take an explicit stance, in case it is not perfectly clear, I am against wearing the dress. Not because wearing it is racist in of itself but because wearing the dress without any implications behind it is a privilege when it should not be.
"This person from X country disagrees with the whole idea!"
This one is easy to shut down. No shit a Japanese person from and currently living in Japan doesn't mind a white person taking on a role written as Asian. In their world that is a novelty. No problems at all with a white girl wearing a kimono. Why would they? They've never been afraid to wear one.
"Then don't be afraid!" Easy words from someone who doesn't have a price to pay for doing it or is willing and/or able to pay it.
"You can't just claim only X race can do X, that's racist!"
This is technically true in the most simplistic terms. But we also live in a world where Y person doing X is accepted or praised where as X person doing X receives backlash. No matter how the situation is played, exclusion of some kind exists. But to focus on individuals rather than society as a whole ignores the power dynamics in place. And to ignore them is at best irresponsible and at worst the perfect example of privilege. Individuals can indeed harm each other but it's societies that cause entire groups of people to experience harm. It doesn't all have to be malicious. Just stack the deck a little and with enough time it'll show its face. The house always wins for a reason.
This isn't exhaustive, nor is it my final word. But it was a shell that needed filling.