A Hyperspace interdiction projector is a shipboard device that allows a ship to force other ships out of hyperspace, and back into normal space. HIPs are often used by military forces at checkpoints along hyperspace routes.
They can also be used to protect assets, such as planets - military vessels would project a hyperspace interdiction field in a sphere around the planet at long range, which would force any ship approaching the planet out of hyperspace, thus giving the military vessels a chance to screen incoming ships, and eliminate threats.
Most large political powers prohibit civilian usage of interdiction devices, although that certainly doesn't stop those who are determined. Pirates make extensive use of interdiction technology, without it they would have to just hope someone drops out of hyperspace, but this way they can sit at a certain point along a hyperspace route, and interdict any vessel that is travelling along that route.