If someone uses the expression 'money doesn't grow on trees', they mean that money is hard to get, or earn. It is not easy to obtain.
You have to work hard to earn money. It does not come easily for many people.
Children often think that money is easy to earn because they see their parents spending but not earning money.
For example, it is quite common when people are young to ask their parents for something, and the parents would reply by saying 'No! We can't afford it. Money does not grow on trees, you know
Here's one more English expression for you to learn.
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