How do icon-fonts generate CSS 'content' code?

Hello everyone,

I was wondering how content code are generated .I was looking at font-awesome icon and noticed the they have 'content' property on pseudo elements.

If i create one custom icon ,how create code for it.

For example Here is the css for fa-file-image-o in font-awesome.

.fa-file-photo-o:before, .fa-file-picture-o:before, .fa-file-image-o:before {
  content : '"\f1c5"
}

How does this code is generated.

If any one can explain.

Thanks