usb card reader?

As mentioned above it depends on the types of files you copied. You could look at the properties of one random file thats hidden and uncheck it's hidden attribute. Of course you will need to enable show hidden files in file explorer.

Hey,

So yes as mentioned above they could be hidden files, or they could either be temporary files which are only created for hardware compatibility, or if you had the card in another device before it is possible that the device generated some temporary files, either way if they seem to be of a problem to you just format the card and put the files on again and use it as you did before.

So, hidden files can generate if the formatting of the device in this case the card is formatted via the computer instead of the device you plan to use it with, and that often causes the files to have a "_" character before their names which simply makes them a hidden file when windows recognises it. Also if the files are indeed starting with a number in this case an album number I would consider changing the name of the file that might fix the issue with it being hidden.