When used in the phrase "get down to brass tacks" and related variants ("see the brass tacks", "pull brass tacks") means to get straight to the point at hand and to not beat around the bush (to use another idiom).
The dervination of the phrase is much debated but is usually tracked down the the furniture trades in the early 1800's where one could tell the quality of the furniture to the type and the number of tacks holding the frame and upolstery together and therefore denoted quality. Using this history it would mean to strip away all outword flourishes and to concentrate on the core of an element.
There are, however, two other contestants. The first claims that it is refering to marks made on a shopkeepers counter to serve as a measuring guide for selling items by length. The third and most contested is that the term 'brass tacks' is cockney slang for 'hard facts' because they rhyme.