This pattern minimizes the memory usage when an application deals with a large number of similar objects. It extracts as much common data as possible to special shareable objects - flyweight. This shared data is called intrinistic data and can be used in different contexts thus a shared flyweight object can be used in different contexts. A context combined with a flyweight imitates a "real" distinguishable object.
So if we had an array of 100 objects sharing mostly the same state then this state could be represented by one flyweight object. Then the flyweight's methods can be passed a context data to represent each of 100 initial objects.
See https://en.wikipedia.org/wiki/Flyweight_pattern for more information.
Imagine a delivery center that charges trucks with packages. Each truck can carry a limited weight of packages. There a two standard type packages - a small one and a big one with defined standard volume - 2m^2 and 10^m respectively. Also clients can specify custom packages with arbitrary volume. All of the numerous trucks contain an array of packages. If each package in the application was represented by its own allocated object it would cost a lot of memory.
The solution is to extract common information - name of a package and its volume into a two shared flyweight objects: an object of SmallPackage and an object of BigPackage. The truck's arrays of packages reference only these two objects. These are concrete flyweights.
There are also custom packages that cannot be shared. Each package requires a separate object that represents it. It's the objects of CustomPackage class. This is unshared concrete flyweight.
SmallPackage, BigPackage and CustomPackage implement PackageInterface.
The latter defines the method getTotalWeight which returns a weight of a
package based on intrinistic data (volume) and extrinistic data (density).
The volume value is stored in the flyweight, the density value is passed to its
method.
Every instantiation of SmallPackage, BigPackage or CustomPackage must be
done via PackageFactory. It assures that the objects of SmallPackage and
BigPackage classes are instantiated only once.