This small crater (approximately 7 meters, or 20 feet, in diameter) was created sometime within the last five years. We know this because we have enough images of Mars over the last decade that we can narrow down its formation date using "before" images where the crater did not appear.
We are able to discover these new craters because the dark area surrounding them is much larger than the crater itself. We can detect this larger dark spot with lower-resolution data sets from other instruments. Then when HiRISE follows up, the higher resolution image reveals the crater at the center of the dark spot, as well as details of the ejecta and surrounding area.
Studying new craters like these will tell us more about the current rate of impacts on Mars. A common technique for estimating the age of a given surface involves counting how many craters of each size are present on that surface. This will help us better understand results from that technique, and refine it to give more accurate ages.
Small impacts like these also represent a hazard to future exploration, both on Mars and the Moon. Imagine if one of the Martian rovers had been at this spot when this object blasted into the surface! This may be a very small crater on planetary scales, but it could still cause a lot of damage. On the Moon, where there is no atmosphere to provide protection from even the tiniest micrometeorite, the danger would be even higher. Knowing the rate of impacts like these will tell us how much shielding will be needed for future landers, rovers, and eventually human explorers.
Be sure to check out one of many more new craters HiRISE is discovering.
Photo credit: NASA/JPL/University of Arizona
Note: This crater is located on the northwestern flank of Biblis Patera, which is a volcano located on the Tharsis bulge, to the west of Pavonis Mons.