What the heck does "get the slack out of the bar" even mean? Well, in short, it refers to getting better leverage and "connectivity" before you initiate the pull.
Many lifters yank the bar off the ground, which in turn elicits a loud "clank" noise (barbell hitting inside of plates). Getting the slack out of the bar means using the barbell as a counterbalance to 1) gain leverage, and 2) get everything connected.
I like to tell my athletes to get two clicks: bend the bar (get the slack out), then pull.