Something like this is not depend on positional understanding. You have to calculate it. You said you want to play d5. But if you do that, black can remove your pawn by playing c6 or e6. Your plan is good, if your opponent cant prevent it, or even if he can, than you wont have worse position. So, usually you have to calculate the moves, positional understanding can help you, when the position is closed, (so there is no forced variation) and it can help you in general, so what should you do with different centre types, with different pawn structures, what should you do if you have bishop pair or play against it and so on. But in an open position calculating is your friend. (But obvious what have to calculate at a move like d5 in this position)
What is and how to understand the difference between a bad move and a good move?
I think you should concentrate on development and king safety. There are a few opening principles you should know and use (minor piece development, king safety, not moving the same piece twice, etc), google them. Moving the same piece twice waste moves and your opponent can easily undermine your d5 with c6 and help his own development as after you take it the knight jumps into the game Nc6
As mentioned, in this case d5 is bad because it allows your opponent to easily attack your centre.
In general it's a bit of a silly question. Finding good moves instead of bad moves is the whole skill of chess.
Something like this is not depend on positional understanding.
Actually it is. Good positional understanding would tell someone not to even consider d5.
In this position, d5 is not good because it is one step closer to "opening" the center but you are not castled yet, so you don't want to open the center yet. Better to play Be2 and castle next move.
You don't have to worry about Nc6 because if your opponent plays that, then you are already castled but his king is still in the center (because he wasted time playing Nc6). At this point, you can think about opening the center with a move like d5. (Because his king is still in the center, so opening the center means lines of attack for you.)
So d5 is premature at the given point in the diagram. You don't really have to calculate anything to see this. Positionally, d5 should not be played at this stage because you are not prepared yet to open lines in the center.
How do I know the difference between good and bad moves?
For example, I would like to talk about this position, 1.e4 d5 2.exd5 Qxd5 3.Nc3 Qd6 4.d4 Nf6 5.Nf3 a6 .
As white I am tempted to play my play my pawn to d5, taking space and preventing my opponent from playing Nc6. I think its a good move, but there isn't a single mention of it in the Game Explorer in Chess.com so it must be bad. However I am unable to recognise that it is not a good move and play it in games. I win some games, because I throw opponents off track, but otherwise I don't.
I don't memorize openings because people tell me not to. However, when I played a WGM and got beaten, she told me I should study openings.
I'm rated 1278 OTB, and I am all about tactics and middlegame.
How do I gain a better understanding of the game so I will be able to understand and recognise good/bad moves?
PS--- I try to avoid what teachers recommend beginners - Play 1.e4 e5, 1. d4 d5 because they are more complicated than what it seems with so much theory. Would playing those moves help me get better understanding of the game?