It fills the entire device with data and then tries to read it all back. It can tell you how many bytes were successfully read, how many were corrupted and how many were written over by other writes.
Even on cards I know are real I still run the test because I have had a card that had a few bytes that got corrupted which caused loads of issues with my rpi.
The test time can be pretty insane on the high capacity cards. I remember it took about 4 hours to test a 256gb card but I find its worth it to check the card just once since it saves a lot of pain later when you find one tiny part of the card is failing.
It fills the entire device with data and then tries to read it all back. It can tell you how many bytes were successfully read, how many were corrupted and how many were written over by other writes.
Even on cards I know are real I still run the test because I have had a card that had a few bytes that got corrupted which caused loads of issues with my rpi.