When you see a filename pattern like The.Rookie.S02E04.OpenH264.[...].mkv , it means that the copy of the episode you are viewing was rather than a different H.264 encoder (like x264, Apple's VideoToolbox, or NVENC).
Some streaming sites trigger a pop-up stating: "The OpenH264 plugin is required to view this content." This means your media player lacks the internal decoding library to process the episode's compression format. Troubleshooting Playback Issues the rookie s02e04 openh264