Mixins obscure the movement of coins on the blockchain, analogous to a Bitcoin mixer/tumbler but far more robust. Multiple ‘outputs’ go in and it’s not clear which outputs are actually being spent, where they came from or where they’re going. The number of mixins is how many other outputs your output is indistinguishable from.
The problem is that if you know the size of an output (e.g. if you created it), you can sometimes link transactions together by following that size along the blockchain.
Monero Research Lab developed RingCT to close off this loophole. RingCT makes it impossible to see the size of an output on the blockchain so you can’t ‘follow the size’ to link transactions. Monero started using this in January. Adolfcoin uses Mixins and RingCT (and makes them compulsory) right from the genesis block.