Misconceptions in 'Doctrine of Conversion'

Can someone please explain 'Doctrine of Conversion' in simple terms with a real-life example? Our textbook just defines it as "a process of converting an existing use into a new one". But it's so vague. What I get is that if I build a small shop in my backyard, and later turn it into a residential space, it's an example. But how? Is it just a matter of putting up a new door and calling it a day? Or is there more to it? I've seen examples online, but I'm still confused. Can we get a clear explanation with Indian context?