Here's a brief explanation of why it's big:
Why is observing gravitational waves such a big deal? Apart from the fact that it would affirm a big piece of Einstein’s theory of general relativity, gravitational waves can be used to probe some of the most mysterious phenomena in the cosmos. As LISA Pathfinder scientist Bill Weber told Gizmodo last month, they’re “the most direct way of studying the large fraction of the universe which is dark.”
Black holes, neutron stars, and other objects that don’t emit light are very difficult to study directly from our vantage point. But gravitational waves, which pass through such objects like a knife through butter, offer us a window. In probing the dark universe with gravitational waves, we might uncover new celestial wonders we never dreamed of.