From the description here and the schematic diagram here, it seems that it's a camera that's capable of recording two images separated by an extremely short (e.g. 5µs) delay. You can see from the schematic that the first image is read out of the device at the same time that the second image is being exposed, so it seems likely that the sensor itself probably incorporates a latch on each pixel that stores the value until it can be read, allowing the entire image to be saved and enabling the very short delay between the end of the first exposure and the start of the second.

The goal of particle image velocimetry (PIV) is to analyze the flow of a fluid by tracking individual particles suspended in the fluid. It's important to be able to take two images in rapid succession so that you can match the particles in the second image to those in the first before they move too far.