The title is a famous quote form Sun Tzu, the famous Chinese military strategist and philosopher. It's from his book The Art of War, but much of it equally applies to sports. This quote capsulizes what should be obvious to table tennis players - that a match is, indeed, won before it begins. Let's look at what goes into winning a match.
First off is your equipment. That is chosen before the match begins.
Next are your techniques, the most important aspect. This means your strokes, serves, receive, and footwork. But these are all developed before the match begins.
Next is the physical aspect. Again, any physical training is done in advance, and other aspects (height, age, health and injuries, etc.) are also determined before the match begins.
Then there's the mental aspect, perhaps the most varied part from match to match. But any mental training (or lack thereof) also comes before the match. Even if you haven't had any mental training, the mental habits you pick up are pretty much set before the match begins. If you are the nervous type who falls apart under pressure, then you were the nervous wreck who falls apart under pressure before the match began. If you are the type who is cool under pressure and plays great when the match is on the line, you were that way coming into the match. If you want to change or improve these aspects, you need to do it before the match.
Finally, there is your tactical game. But again, your tactical skills (or lack thereof) were developed before the match. If you have a coach, that is normally arranged before the match. And if you want to scout your opponent out before the match starts or talk to someone who knows the player, and develop a tactical plan, that too takes place before the match.
Now anyone can get technical and point out that no matter how much preparation you do there is still some randomness - nets and edges, how well the opponent plays, even the accuracy of advice given from others. But while these are unknowns going into the match, it is your preparation before the match that sets the chances of your winning, given the circumstances. This includes the mental aspect, the part that most varies from match to match for most players. Players are often stronger mentally in some matches than others - which really shows a mental weakness in the times when they are not mentally strong, something that can be overcome with sports psychology training. But the preparation before the match sets the odds of being mentally strong in a given match. Perhaps the article could better be titled, "The Odds of Winning a Match Are Set Before the Match Begins." But in the long run, it's the same thing.
So the next time you are getting ready to play a tournament, don't worry about how well you'll play. Worry about how well you've prepared.