All these variables are prefixed with m_ because they are class member variables.
This is an MFC naming convention. After the m_, a form of Hungarian notation is used, in which the next few letters describe the variable type. In this case, b means boolean, and str indicates that the variable is a string.
You’ll see this naming convention in use in this book and other books about programming with Visual C++ and MFC. Following this naming convention will make your code more readable for other programmers; knowing the convention will make it easier for you to read other programmer’s code as well.