Remember the old video game Pong? Pretty much the same principle.

You want to hit it between the lines on the other guy’s side, in such a way that he can’t hit it back to you. If it goes on the line, it’s considered “in” but outside the line is “out”.

Tennis scoring is what is so wacky.

Only the serve has to land within a particular zone (one of the two boxes just beyond the net). Once the ball is in play, it can land anywhere within the court. For singles play, the narrowest set of sidelines marks the bounds. For doubles play, the widest set of sidelines is used.

Okay, you have the service boxes in which to place your serve.
Then there’s the baseline, where rallies frequently end if the ball goes long.
The tram lines run along the side of the court & the ball must ground within these, except for in doubles play, where the line furthest wide comes into effect.

The ATP experimented with leaving the doubles tramlines off the courts, in tournaments where there were only singles matches to be played.
What a difference, the courts looked absolutely tiny, an optical illusion that made no impact on single play, but a huge aesthetic change, not a good move.

Thank you all for the answers.