It's actually not different than believing in hope. It means that we believe something good will come if we have faith in it. People define positive thinking as a successful result from our actions on the situation we're in. Positive thinking can be the greatest key to success.
But of course, not everyone believes in positive thinking. Some people just consider it nonsense or a superstition. When people tell them to think positive, they don't take it seriously. These are the negative thinkers.
Negative thinking means that we would expect a horrible result from our actions. This way of thinking limits our true potential. For example, Z believes he wouldn't get a good result on his exam. So he didn't study and prepare himself enough for the exam. He earned a C and thought to himself, "I could've gotten a better grade!" This kind of thinking can only get us so far, even though we could've actually gotten further.
How can we be happy if we have negative thinking? A negative mind brings us sadness and anxiousness. What benefit is there from believing misfortune will come? To be honest, A positive mind doesn't always bring us good result. Maybe our actions were wrong, or we didn't do well enough. But having a positive mind will bring us happiness in the process, and motivates us to do better. That's surely better than believing we're going to fail or end badly. So convert our ways of thinking into something more positive and motivating.
It takes process for us to convert a negative mind into a positive mind. We can start by placing positive or supportive thoughts in our head. Imagine the benefits of our actions and motivate ourselves. Throw away all our desires to give up and quit. Then our minds will slowly change into a positive thinker.
Just look at how much we'll be able to achieve from a positive mind!