Often nearly impossible to see, a skimmer is a small device placed in or over a card reader (ATM, gas pump, checkout) to record information from your credit or debit card's magnetic stripe. That information can then be used to make fraudulent purchases and sometimes even a clone of your card.
While most new and replacement credit/debit cards you receive will have a chip (metal square on the front of the card) that offers better protection against skimming, some businesses (gas stations) still use the magnetic stripe reader. Chip readers aren't perfect either. If a reader requires you to fully insert your card, a skimmer could still be used to read your card's magnetic stripe. More advanced skimmers (shimmers) have also been developed to steal your information directly from the chip.
It is impossible to protect yourself fully from skimming and shimming, but you can reduce your risk by checking for signs of tampering on card readers and monitoring your accounts closely for unauthorized charges.