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I think he tries to understand what the code in the tutorial does.

double a = cos(theta), b = sin(theta);
double x0 = a*rho, y0 = b*rho;

Is just the transformation from polar coordinates to Cartesian coordinates. This is the point where the blue and the red line meets.

The next four lines of code "calculate" the points x and y, which are then used to draw. I wrote "calculate" because they don't really do, but just move a 1000 pixels in both directions, horizontally and vertically. If you have an image much larger than thousand pixels, you'll find that most lines won't reach the outer borders of the image but end somewhere with a total x-distance of 2000 pixels and a total y-distance of 2000 pixels from end to end. However, all lines include the point (x0,y0), th e one where blue and red line meet. From this point to x, deltaY is 1000 and deltaX is also 1000, same is valid for the distance between this point and y, just deltaY is -1000 and deltaX is also -1000

I think he tries to understand what the code in the tutorial does.

double a = cos(theta), b = sin(theta);
double x0 = a*rho, y0 = b*rho;

Is just the transformation from polar coordinates to Cartesian coordinates. This is the point where the blue and the red line meets.

pt1.x = cvRound(x0 + 1000*(-b));
pt1.y = cvRound(y0 + 1000*(a));
pt2.x = cvRound(x0 - 1000*(-b));
pt2.y = cvRound(y0 - 1000*(a));