Canon cameras use an SD memory card to store images. Locking the memory card prevents images from being erased. This is helpful if you are using multiple memory cards and don't want to accidentally erase or overwrite pictures you've already taken. You will get an error message if you try to take pictures on a locked memory card. If you want to use the card again, you will need to unlock the memory card.
Turn off the Canon camera, and open the side compartment that stores the memory card.
Press the black eject button to remove the memory card from the camera.
Flip the plastic switch on the side of the card so it is in an upward position. This is the unlocked position.
To lock the card, flip the switch on the side of the card so it is facing down.