Uff, that's a tough question. One that I've been pulling my hair out for over 6 months.
But here goes nuttin'.... Just what I've read and gathered from my own experience. I'm not a pH whiz, so bear with me.
I've read the magical number for pH in wine is 3.5 or so. If your 0.3 or 0.4 in either way, it doesn't matter. It's still okay.
pH is something that tells you the "sourness" in the wine. If it's LOW, that means acid is HIGH. (2.5 means lots of acid, conversely, 4.5 means it's LOW in acid).
If the pH is in the range, it is usually universally liked by all (not talking about theingredients). If it's higher or lower, you'll probably get some critical comments about it. I personally love wines that are a bit more sour than most, so it's also a personal preference.
I have started to use my pH-meter to test commercial wines that I buy and see what I like. It seems to be going well.
Furthermore, the thing to remember is to take a pH reading of the must and the finished product. If you need to correct the must, it can ward off bacteria and whatnot better with the correct pH. What's the "correct" pH or the target pH of must? I haven't figured that out yet, and don't remember where I could find it.
Hope this helps to get you started....
Martina