What does `m_` variable prefix mean?

This is typical programming practice for defining variables that are member variables. So when you're using them later, you don't need to see where they're defined to know their scope. This is also great if you already know the scope and you're using something like intelliSense, you can start with m_ and a list of all your member variables are shown. Part of Hungarian notation, see the part about scope in the examples here.

It is common practice in C++. This is because in C++ you can't have same name for the member function and member variable, and getter functions are often named without "get" prefix.

class Person
{
   public:
      std::string name() const;

   private:
      std::string name; // This would lead to a compilation error.
      std::string m_name; // OK.
};

---

main.cpp:9:19: error: duplicate member 'name'
      std::string name;
                  ^
main.cpp:6:19: note: previous declaration is here
      std::string name() const;
                  ^
1 error generated.

http://coliru.stacked-crooked.com/a/f38e7dbb047687ad

"m_" states for the "member". Prefix "_" is also common.

You shouldn't use it in programming languages that solve this problem by using different conventions/grammar.

In Clean Code: A Handbook of Agile Software Craftsmanship there is an explicit recommendation against the usage of this prefix:

You also don't need to prefix member variables with m_ anymore. Your classes and functions should be small enough that you don't need them.

There is also an example (C# code) of this:

Bad practice:

public class Part
{
    private String m_dsc; // The textual description

    void SetName(string name)
    {
        m_dsc = name;
    }
}

Good practice:

public class Part
{
    private String description;

    void SetDescription(string description)
    {
        this.description = description;
    }
}

We count with language constructs to refer to member variables in the case of explicitly ambiguity (i.e., description member and description parameter): this.