On one computer screen the length of a centimetre might be 200 pixels, whilst on another it might be 400 pixels. The thread writer is suggesting that a sf::Distance function would be useful to applications that want to provide a to scale drawing of something. For example:
sf::RectangleShape box;
box.setSize(sf::Vector2f(sf::Distance::Centimetre * 2, sf::Distance::Centimetre * 2));
box.setPosition(sf::Vector2f(500, 500));
box.setFillColor(sf::Color::Red);
The above code would create a box that would be exactly 2 by 2 centimetres.