This refers to High Definition (HD) resolution (1280x720 pixels). It offers a strong balance between sharp visual quality and smaller file sizes.
Before we discuss where not to watch it, let's look at the movie in question. "6 Underground" is a 2019 American vigilante action thriller directed by Michael Bay and written by Paul Wernick and Rhett Reese. 6 underground 2019 720pmkv filmyflycom best