What is uncompressed?

Some articles on uncompressed:

Practical Configuration and Recording - Uncompressed Video Recorder
... recorders are an easy, reliable and complete solution for receiving and storing uncompressed video ... Partly they receive uncompressed video, but only record lossy compressed video, often in the lossy Apple ProRes 422 or DNxHD codecs ... Professional recorders support multiple channels of uncompressed HDMI, DVI or (HD-)SDI recording, but are limited by the total data rate ...