the most fundamental considerations; essentials; realities (usually used in the phrase get down to brass tacks).
Origin of brass tacks
First recorded in 1895–1900
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
informal basic realities; hard facts (esp in the phrase get down to brass tacks)
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012