Yes, rassolnik is a pickle soup. It's a traditional Eastern European dish, quite popular in Russia and Ukraine. The main ingredients are brined pickles (or pickled cucumbers), pearl barley, and various root vegetables like potatoes, carrots, and onions. It often includes meat, usually beef or offal like kidneys. The sourness from the pickles and their brine is what defines it. It's not "wierd" at all, it's a hearty and flavourful soup, especially good in colder weather. Your friend likely enjoys it; it's quite a staple over there.