What is the meaning of 1:1 in unity?

I’ve read in a few sites like this and weblogs that learn unity 2d, when they speak about orthographic camera, actually say about the 1:1 ratio. I can’t catch what is the use of this concept? can anybody explain it to me?

For every 1 pixel in a graphical object you will be representing on-screen, 1 pixel is taken in the camera view. IE: Pixel-perfect camera.

Zoomed out, you may lose fine detail. Zoomed in you may end up with blurry textures.