English Only » meaning of "referred by"

Hi guys,

on one of the application forms there is a field named "referred by" .
In german i would understand it as "bezeichnet" or "genannt" + "von".

What do they really mean in application forms, i've never seen the by in combination with referred.
Do they mean how my profession is called or maybee a recommendation what does it really mean?

Thanks
Carlos

edit:
I've tried to figure it out on http://www.thefreedictionary.com/referred
But the explanation is not clear enough for me.

Ok i think they mean how did i find them ,what kind of job search engine or something like that.

Is it any slang because this kind of refer usage sounds a bit strange to me.