Question about cos and sin in math function

I'm using OpenGl, and when I'm studying, do #include<math.h> and define #define PI 3.141592 in the code.

    glBegin(GL_POINTS);
    for (double angle = 0.0; angle <= PI * 2.0; angle += PI / 30.0) {
        glVertex3f(0.5 * cos(angle), 0.5 * sin(angle), 0.0);
    }
    glEnd();

After writing for statement like this,

Here's the result. I don't know what the content of that for statement means because there is only a trigonometric function related to cosin that I know. What should I refer to in order to understand that phenomenon? What else should I study to understand that phenomenon? For your information, the other codes above are codes that mark the coordinates specified in the for statement, and the content of the for statement is about the coordinates where the dots are marked.