[Koha] Koha for beginners

Hi, I am new to Koha and have no library science experience. I am trying to understand MARC but stuck at the terms where there are some numbers with a dollar sign and then an alphabet. What is this basically? I would be grateful if someone can help me with a book or link that helps understand these terms or library/koha terms in general or in easy comprehension. other thing that i want to know is if a book is bilingual (Spanish and English or Arabic and English) , what should i write in Lnaguage code clumn? Thanks *Mehvish*