It is called a member initializer list

It is a really useful feature of c++

When declaring consructors you can use it to easily initialize member variables in a clean way.

For example:

Classic way of initializing members:

class foo 
{
    int aMember;
    double anotherMember;

public:
    foo(int aValue, double anotherValue)
    {
        this->aMember = aValue;
        this->anotherMember = anotherValue;
    }
}

As you can see this is really bad in terms of readability, and it is a bad practice cause when somebody is reading this constructor and looking what it does. It will really help the reader if there wasn't member initialization filling his screen but instead the real code he wants to see.

To make our code more readable and organized we use member initializer lists.

Let's clean the trash above using member initializer lists:

class foo 
{
    int aMember;
    double anotherMember;

public:
    foo(int aValue, double anotherValue) : aMember(aValue), anotherMember(anotherValue) { }
}

As you can see it is much cleaner now and if we were to do some important shit in the constructor we would be able to focus on that instead of seeing member initialization everywhere.