What is singleton and why we have to use

The idea of a singleton is very simple: it's a class with a private constructor, so it cannot be instantiated form outside the class. This means that the class is absolutely in control of how many instances of the class are created, and when they are created. A singleton class creates a single instance of itself, which it returns to calling classes that need to access it via a specific method for that purpose.

Personally, I think of them as a glorified global variable which kinda breaks the whole OOPs paradigm, and don't use them - but I have seen them used well for single-resource systems where there is one hardware object which needs to be used sensibly by multiple classes and /or threads.

Generally considered an anathema these days ...