What does a function inheriting from a private variable mean (C++)?

Question

I have come across a few examples while researching the Poco Net library, and have no clue what this is called, or why it is even used:

class TestClass {
  public:
    TestClass() : _length(0){}
  private:
    int _length;

I've got 2 questions about this:

1. How is the function inheriting from a variable, and

2. why does the variable have a parameter (0)?

Thank you.