Object instantiation in C++ with type

May someone tell me please why we do write size_t after Graph in the following line?

Graph g;

Graph is a class name and g is an object. What does size_t do there? Why should we write that?

I am so sorry if the question is too basic. But I could not find explanation on this and so far when I've wanted to create an object of class I have written:

class_name object;

like:

Graph g;