Dack is a Rebel pilot assigned to be Luke Skywalker's gunner during the Alliance's efforts to defend their base on Hoth from the forces of the evil Empire. In many ways he's similar to Luke himself; young, energetic and eager to take the fight to the villains. "Right now I feel I could take on the whole Empire myself!" he assures Luke as they prep for battle.
Sadly, Dack doesn't make it through the fighting. Problems arise with their snowspeeder's firing controls early on.
First, the lack of an approach vector prevents Dack from being all he can be, and then a malfunction in fire control causes the entirety of Dack's control panel to explode. Either that, or a stray blaster bolt hit the snowspeeder; the film is annoyingly unclear as to just what happened.
In addition, it's also unclear if the explosion kills Dack or merely wounds him and leaves him unconscious. In any event, he's out of commission, preventing Luke from directly participating in the battle and instead flying escort for his fellow pilots Wedge Antilles and Zev Senesca, the latter of whom gets blasted out of the sky fairly quickly. And then so does Luke.
Luke's snowspeeder crashlands violently in the snow, but Luke survives. Seeing the Imperial AT-AT approaching them, Luke opens the canopy and turns in an effort to pull the possibly dead Dack free as well.
However, he is unsuccessful, and, turning and seeing the AT-AT bearing down on them, Luke is forced to make a split-second decision, abandoning the most likely dead, or at least wounded, Dack as the walker's foot comes down upon the snowspeeder, crushing the downed craft underfoot.
It's easy to hold it against Luke for leaving Dack, but sometimes heroes have to make tough decisions with very little time. If he'd lingered and kept trying to pull the probably already dead Dack free, he would've been crushed to death himself. But fear not, Dack fans (if any exist); if it's any consolation, Luke puts paid to the walker that squished his friend a few minutes later when he plants a bomb aboard the offending vehicle and blows it to bits.