From level 711 you will encounter KEYS.
The keys unlock certain locked cells when you match them as you would a normal candy.
Sometimes just matching one key will open any locked cells, but on some levels you need to collect several keys to open the locks.
Sometimes it's easy to find the locked cells, but other times you have to look carefully to see what it is you're unlocking.
Usually on ingredient levels the keys will unlock the dispenser where the ingredients will drop from and it may take several keys to finally release the fruit.
Here's what the locked cells look like.
These locks need multiple keys to open them, each time you match a key one layer will be removed from the lock until it is finally open and the ingredient released.
The image below shows locks which only need one key to open them.
Below is a video of a level showing the keys and locks in action. I used a colour bomb in this video to hit the key, but they can be destroyed in all the same ways as a normal candy, either by matching, hitting with a stripe or any combo.