displaying icon image inside content property in IDE

Question

the picture of code, in "content" property, it's value is a shape! , as you see in above image, specific part of my code! I have style sheet in a project (that has been written before by another) that it have some css codes, and inside that, like bellow, have a property that, inside its value, it shows icon image! i cant understand that, how inserted/created that!

my code:

.o-icon--close:before {
content: "";

}

my problem: how he/she has written/created that icon shape inside CONTENT property. it isn't a copy/paste an icon shape. it isn't a text code!

as here isn't IDE, it shows a vertical rectangle, but in IDE it is shape of icon itself! like a music sign or what ever. i know that, for CONTENT value, we can use fontawesome codes or other texts, but i can't understand how insert icon itself as a shape!