The dune stands between the rover and a small valley, with scientists at NASA preferring to go over the dune rather than taking other routes that could be riddled with sharp rocks. Curiosity's titanium wheels have already taken a beating, with recent pictures showing multiple rips and punctures.
NASA wants to drive through the 'Dingo Gap' to protect the rover's wheels, and is confident Curiosity can make the short trek. While the rover is capable of traversing the dune, scientists are worried it will get damaged or snagged by hidden rocks. So in order to be safe, engineers are commanding the rover to go only part of the way up before deciding whether the rover can make the entire journey.
Curiosity is attempting to reach Mount Sharp, which is still miles from Curiosity's current location in the Gale Crater. Scientists are attempting to drill into the exposed bedrock and look for traces of complex carbon chemistry. Curiosity has so far clocked 3 miles since landing on Mars in 2012.
NASA lost the Spirit rover in 2009 in a sand trap, and drove the Opportunity rover into a deep pile of dirt in 2005, where it lay for several weeks before breaking free. The Opportunity rover has since recovered and recently celebrated 10 years of work on Mars.