Nanobots are emergency items similar to Shield Batteries. While Shield Batteries restore Shield Points (usually by bringing shields all the way back up after they've been knocked down), Nanobots restore Hull Points. It is noteworthy that Nanobots cannot repair damaged or destroyed equipment, and cannot replace destroyed sections of one's ship (such as a lost wing or stabilizer). To repair major structural damage or damaged equipment, one must land at a base and visit the repair bay.
Nanobots cost $200 each, and seem to drop less frequently from most enemies than Shield Batteries. Nanobots can be used to extend one's stay in radiation zones, which eat the hull of your ship without touching your shields. Don't forget your lead undies!