These indicate a video format that has 50 or 60 progressive frames per second. The original HD digital television standards only included progressive frame rates above 30 Hz for image sizes up to 720 lines. It was later extended to the larger 1920 x 1080 television standards to provide a fast frame refresh rate for the rendition of fast action and progressive frames for optimum vertical resolution (better than interlaced scans).
The baseband signal produces twice the data rate of the equivalent interlaced (50I and 60I) formats, pushing up equipment specifications.
See also: SDI (3G 6G, 12G, SDI)