why use "static" keyword before public data member?

I have an integer data member called "students" in public part of my class and is used as an argument in the constructor. gradebook.h:

class gradebook
{
public:
    int students = 10;
    gradebook(string, int[]);
    void SetCourseName(string _coursename);
    ...
private:
    string coursename;
    ...
};

gradebook.cpp:

#include "gradebook.h"

gradebook::gradebook(string s1, int array[students])
{
    SetCourseName(s1);
}
void gradebook::SetCourseName(string _coursename)
{
    coursename = _coursename;
}

when I try to compile the code receive this error:

invalid use of non-static data member 'gradebook::students'
gradebook::gradebook(string s1, int array[students])
                                          ^

if I put 'static const' before 'int student = 10' the problem gets away.

why?