It's a device that you plug into a usb socket on a computer, then inserts the microSD card into it, and the computer can read the SD card. They are pretty inexpensive.
Few computers come with a socket for reading microUSB directly, which is why I suggested a card reader (though many have full size SD readers, so if you have a microSD to SD adapter, which some cards come with, that would be another possibility).
But if you don't know what a card reader is I doubt you have one. So have you tried removing and replacing the card?