Basically, lac operon is an enzyme, and enzymes work in a very well known way called lock key system
. This lock key system works pretty simply, imagine the lock as the enzyme, and this lock needs a specific key to open it, thinking about an enzyme, it needs the specific substrate to breakdown the lactose, this enzyme is called lactase, and it's present in some animals and bacterium too. This doesn't only work for lactose, it's the same lock system for every enzyme, all of them need specific substrate to break it down into things that the body can use for many things.
