I found a topic in tangents and normals which called angle between two intersecting curves. When I turned my page. I saw that they had used tilde in most of equation. But, I totally don't know how tilde works. Does it work like negative? What does this tilde mean in intersecting curves (equation)? Without knowing it I can't understand any equation. I was seeing some problems. But, I don't know what they did to tilde.
$$\theta = tan^{-1}\frac{f'(x)\sim F'(x)}{1+f'(x)F'(x)}$$