When people say they're "fake," they mean that some random Chinese company (or whatever) manufactured a micro SD card to look like a Samsung one, a Lexar one, a Sandisk one, etc.. The problem with those fakes is that they tend to be unreliable because they're so cheaply made. So basically, there's a high failure rate, which for you, means that you can lose all your data because the card is cheap and will stop being read by your Switch (or whatever device you put it into).

The way the card "fails" is when it gets too hot and physically burns out. (You can see burn marks on it. The shiny "gold" plating on the underside of the micro SD will turn to a dull, non-shiny brown... sometimes even BLACK.) If you're using something like a Switch, that card is being read constantly and thus gets a little warm. The amount of heat a fake card can tolerate is much less than a legit card. And if you want to see for yourself (once you get a micro SD card), just play a game on Switch for an hour, turn off the Switch, and right away open up the micro SD slot. You'll feel that the card is warm to the touch. A fake card would feel VERY hot, almost to the point it burns your fingers.

I would say that you should only buy microSD cards from places like B&H, Best Buy, Target, and Wal-Mart (but make sure on Wal-Mart that the item says it's "shipped and sold by Wal-Mart").

Also, Amazon has been having issues with people getting fake DualShock 4 controllers and Xbox One controllers, so I just buy from B&H or Best Buy instead. It may cost more money, but at least I know they're not fake. So I really avoid buying any kind of video game hardware from Amazon. I'll only buy games because other video game stuff is hit or miss with Amazon. Sometimes you get legit stuff, sometimes you don't.