glBegin() is a function that indicates we'll defining the vertices of a primitive in the code below. When we're done defining the primitive, we use the function glEnd().
GL_LINES is a macro that indicates we'll be drawing lines.
glVertex3fv() is a function that defines a vertex in space, there are a few versions of this function, so for the sake of clarity let's look at how the names are constructed:
glVertex: a function that defines a vertex
glVertex3: a function that defines a vertex using 3 coordinates
glVertex3f: a function that defines a vertex using 3 coordinates of type GLfloat
glVertex3fv: a function that defines a vertex using 3 coordinates of type GLfloat which are put inside a vector (tuple) (the alternative would be glVertex3fl which uses a list of arguments instead of a vector)
