Go is all about being more efficient with your moves than your opponent. There are three main ways to do this:
1) Play better, bigger moves
2) Play multi-purpose moves
3) Get free moves
Number three is part of what the middle game is about. Force your opponent to defend so that you can get another move in to achieve your objective. So when you reach the middle game, always, always, ALWAYS look to weak groups. If your opponents have weak groups, attack them in a way that helps you. If you have weak groups, find an efficient way to stabilize them. The best way, again, is to attack your opponent's weak groups from the right direction.
The opening is about establishing groups and vying for the advantage of terrain. The endgame is about calculation and taking the most leftover points. The middle game is all about life and death. The more you can push your opponent's groups around, the more you should gain from it, the closer you should be to victory. If you're the one with weak groups the same applies against you. This is what the term 'overstretching' is about.
There's a lot to consider. But at your level, start by taking away your opponents' eyespace and pushing their groups around. As you get stronger you'll find different methods, different timing, strategies, etc. The general rule of Go is that if your opponent's group is not unconditionally alive, you can milk it for points.
So to make a long answer short, threaten your opponent's groups and strengthen your own. That's what the middle game is about.
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stone offers some support, but the cut is still questionable.
is good, as the proverb states.
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gives White a good shape (extending from a crosscut). Not recommended, as a rule.
. He could descend to 5 instead, for instance. The 
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