Coach jargon for the ball right after your serve, the third shot of the point. Roughly 70% of all points are decided inside the first four shots, which is why that ball gets drilled.
The phrase turns up in every coaching briefing and never gets explained: serve+1 is the first ball after your serve. It matters because roughly 70 percent of all points are decided within the first four shots, while only about 10 percent survive nine shots or more. In practice that means you are not serving to hit an ace, you are serving to push the return into the place where your forehand is already waiting. The forehand produces about twice as many winners as the backhand, which is exactly why good players deliberately run around their backhand. The typical mistake is standing still after the serve and waiting to see what happens: then the returner is running the pattern, not you. What often gets missed is that all of this needs a serve you can place. Pace without direction creates chance, not a pattern.