The Grammarist says that the expression “…is an Americanism dating from the 19^th century,” and that it means “the essentials” or “the basic facts.” Both the Grammarist and the Phrase Finder observe that the origin is unknown, but it might have something to do with the brass tacks used in upholstering furniture. The preferred origin for the Phrase Finder is that haberdashers measured cloth between brass tacks set into a shop’s counter.  That source says the meaning is to “Engage with the basic facts or realities.” I’ll add that I think of the expression being used as an admonition to get down to business. “Let’s quit wasting time and get down to the brass tacks.”