Basically you need to hit the batsman with the ball in the line of the stumps for it to be given out LBW (when the batsman is playing a shot) - at the end it shows you the path of the ball - you need all three of those elements to light up red for it to be out and overturn the umpire's decision.I'm not grumbling about anything, just curious of the rules etc.
Usually on TV they'd show something like this - where you see both that ball path and the batsman - you can see that it hit him on the pad, but on a part of the pad that wasn't in front of the stumps.
To be given out LBW, it needs to hit a batsman when their pad (or leg) is in front of the stumps. If it doesn't, even if it was going on to hit the stumps it is given not out.
That's an extraordinarily close call that one - but the game gave the correct decision.