It will lower the rover until Curiosity's wheels are on the surface. Then, more explosive bolts will fire, separating the tethers from the rover. The descent module will fly away while Curiosity begins its sojourn on Mars.
There are a lot of moving parts in this EDL sequence, and a lot of places where things could go wrong. Like the bolts that will sever the tether from the rover as Curiosity contacts the surface. Though there are redundancies in place, if just one cord fails to separate the descent module could drag Curiosity to an untimely death across the Martian surface.
The reality of spaceflight is that things can go wrong. Engineers are only human and freak accidents do happen. Mars Polar Lander's story is a perfect example of how one little thing can make a big difference, turning an expensive mission into debris.
The Mars Polar Lander (MPL) launched on a Delta II in January 1999. When it arrived at Mars in December, it used a heatshield and parachute in the first stages of its descent. It's legs were designed to snap out of their folded position then retrorockets, guided by a landing radar, should have enabled a soft touchdown near the edge of Mars' south polar cap. MPL was designed to collect sampled and analyze them for insights into the planet's surface materials, frost, weather patterns and interactions between the surface and atmosphere over time.