Fruit concentrate is just fruit juice- but stronger. The idea is to take all of the flavor of the fruit and remove all of the water from it. The most common use I've seen is to freeze and store fruit juice compactly, but it can also be used for other things. For example, I made this fruit concentrate so that I could add stronger flavor to fruit juice gummy candies.
There are two ways to make juice concentrate from fruit. They depend on the type of fruit you are using.