Pretty interesting footage, for anyone who has no clue what's going on, the Su-35 (Fighter jet) has launched 3 active-radar missiles towards a target. "Active" means that the plane itself guides the missile in to a certain range, then the missile itself has a radar seeker which can find the target.
(Sidenote, the R-77s can also use INS guidance to get most of the way, to the last known track of the target)
The top of the HUD shows the plane's heading, around 200 degrees at the start of the video. The target has a heading of around 190 degrees. The target is that green diamond with a "1" next to it - meaning it's the first/only target his radar is tracking.
On the left of the screen, you can see a vertical bar going 0, 20, 40, 60, 80 - This is the "range bar" and iirc refers to the kilometres from the target - so this target is around 60km away.
Attached to that vertical range bar, is a smaller green rectangle with a lil arrow next to it - this rectangle represents the range at which a missile can be employed, with different sections of the rectangle corresponding to kill probability.