What does “Turn a lemon(s) into lemonade” exactly mean?

This is a rather popular, commonly used phrase (in many different variations).

No matter the exact wording, in general you want to keep in mind two things:

1. Lemons are sour.
2. Lemonade is sweet.

To make lemonade out of lemons is to take something that is not so great and turn it into something good.

A very common phrase is as follows: "When life gives you lemons, make lemonade."

It means to take a situation that is negative and turn it into a positive. To add sugar, if you will, to a sour situation, event, issue, etc.

It was said about H J Whitley, the Father of Hollywood, who turned lemon orchards of Hollywood into the film Capitol of the world.