Someone please explain calorie deficit?!

I have been on this website for quite sometime. I know what I need to do to lose weight, like I know I need to stay healthy and exercise, etc...

But... the one thing that I really can't quite understand is the calorie deficit. Like... how exactly do we calculate this and how do we know if we've burned 3,500? Since that apparently is how much you burn to lose a pound of fat.

Can someone please explain in laymans terms so that I can understand? Like in detail? Or can someone direct me to a link/thread that explains it. I mean, I know how to use google obviously but I can't find anything that explains it to where I understand it all the way.

Much appreciated!