Abstract class init in the initialization list

I want to understand the following c++ concept. class_a is abstract class and as per abstract class concept we cannot create any instance of it. i have used the initlization list and abstract class as well but never used following concept. In the code ,the initlization list of class_b, class_a is initlized. I want to understand what is meaning of initlizing it in the initilization list.

 class_b::class_b(int val):nameA::class_a()

in fileA.cpp

namespace nameA
{
class class_a
{
    public:
        virtual ~class_a(){};
        virtual void process()=0;
};
}

in fileb.h file

namespace nameB
{
class class_b  : public nameA::class_a
{
    public:
    class_b(int val);
}
}

in fileb.cpp file

namespace nameB
{
class_b::class_b(int val)
:nameA::class_a() //Want to understand this line...
}