Game sense in a broad description for knowledge about your positioning, and each player's positioning, your ultimate status, each other player's ultimate status, your ammo reserves, each player's health, your team's strategy, your opoonents' strategy, your opportunities to make impacts to the game, timing, momentary risk evaluation, and probably a few other things.

If I see someone's VOD and tell them to improve their gamesense, I want them to improve upon their situational awareness at any given moment during the game so that they can develop a sense for how the game plays out around them and how they can influence it to the best of their abilities. Someone withput game sense will often feel helpless, surprised, uncoordinated and blame failures to "bad luck" when in fact all information needed to avoid that failure was right there in the game but it wasn't extracted and understood.

"Bad luck that Rein had Shatter and flanked us when we came out of spawn." No, you could have noticed that Rein didn't ult for a while, that he dealt a lot of damage last fight, that your attacking timer is running low and your team getting wiped lends the enemy a win, you could have checked common corners and noticed that there is no Rein in front of the enemy team. With good gamesense, some or all of the above would have helped you to asess the situation and make a better decision.