It represents the change in how long the asteroid will take to complete an orbit around the sun. It might not sound like much in the context of years-long orbits, but if this change in orbit can be enacted early enough and in the right direction, it can be the difference between the asteroid eventually striking Earth directly or missing it.

Think about a car driving down a road at 60mph. Let's say that 60 miles down the road, someone will walk across the road, let's call them Earth, and they won't be paying attention to traffic, idk maybe they're on their phone and have earphones in. In 1hr, the car will cross paths with Earth and run them over assuming it doesn't slow down. If you try and slow it down right as it's gonna hit Earth, you'll have to really slam on the breaks, and even then it might be too late. But if you intervene 1hr in advance and slow the car down ever so slightly such that the car arrives at where Earth crosses the road just a few seconds later, the crisis is averted with very little energy spent.

That's the exact same principle behind asteroid redirection efforts like this. Hope it helps.