3DES is an alteration on the original DES algorithm, where the input is first encrypted, then decrypted, and encrypted again (with three different keys in the most secure case). This means that the key size can be much larger than raw DES and renders it much harder to attack. 3DES is also backwards compatible, since by using the same key for all three operations, the first encryption and decryption operation cancels out, and only the last encryption takes effect. Do note that while the key size is 168-bit (when all three subkeys are different), the actual strength is only estimated to be 112 bits (@houtvenCrypto1012017).
Do not use 3DES!
Although 3DES is not practically breakable when all three keys are different, it is still not worth the risk. As computing power grows, 3DES will only be a sliver away from security compromise, while alternatives (e.g. AES) still has a wide margin. Plus, it is no where as performant as the alternatives.