a code, which tells our website where who's downloading our pictures

The term "referer" refers to the initial website on which a visitor clicked a link in order to arrive at your website. Complicated? OK, for example, if I was on the BBC News website and there was a link to your website, if I click that link then the BBC News website (and the individual page) will be logged as a referer. This is clearly useful because you know which sites have linked to you, and when you get a referal from a search engine, etc.

The referer field is a field that each HTTP request pass, and it is logged by most log formats. This field tells you where the request comes from. If your URL was manually typed or was called from a bookmark, the referer field will be blank ("-"). If the hits was made from a clicked hyperlink, the referer field will contain the URL of the page containing that hyperlink.