Coin control is a technique used to have full control over your coins. 

You may not know this but if you have multiple BTC addresses on your wallet then, when you send bitcoins to someone else, the bitcoin wallet you’re using chooses randomly which of your wallet addresses will send the coins out to the receiver. 

When you use coin control you choose which of your addresses will send, and even more specifically, which of your unspent outputs will be the sending inputs.

Remember, if you value privacy, no matter how much you mix your coins, you still need to learn to use coin control -- otherwise you’ll be de-anonymized.

Coin control helps you avoid address reuse too, simply be putting the responsibility of controlling your coins back into your hands. 

After all, the fundamental privacy technique (started way back in the Bitcoin whitepaper) is to control your coins by controlling which addresses they flow in and out of.

So if you value security and anonymity, we encourage people to maintain multiple addresses, rather than storing all coins in a single address.

Practicing coin control can be done by watching Youtube videos and reading articles on the subject. The BitcoinQt wallet is one such wallet that lets you practice coin control. Consider using it if that’s something you find important.