During a deadlift, you risk hurting yourself if you let your upper back round from the strain of lifting the weight off the floor. The trick is to keep your body braced and "tight", so to do that, imagine yourself making juice with your armpits. It sounds crazy, but it works. Your upper back can round when you do a deadlift for a number of reasons. Typically, it's that the weight is too heavy, your upper back muscles are weak and/or you're not keeping your body "tight" enough. Tony Gentilcore, a strength and conditioning coach at Cressey Sports Performance, suggests you pretend that you're squeezing an orange in your armpit during the entire rep, like you're making orange juice.
The tightness is crucial because it provides stability throughout the lift but also helps protect your spine. The video above from the strong gals at Unapologetically Powerful demonstrates how to incorporate the cue from start to finish.
Three Better Cues for the Deadlift [Lose Stubborn Fat]