A one swing plane is a description given to the club that rises on the backswing and falls on the downswing on exactly the same line, or at exactly the same angle. This can be demonstrated using an alignment pole.
Place the pole into the target line so that it comes out towards you at about 45-55 degrees. Now stand about two metres to the left of this pole (for right handed golfers). For a one plane swing, you will see the club moving up at the angle of this pole and then back down at the angle of this pole. The hand and arms do not need to manipulate or influence the movement of the club, the swing is driven by good core strength and by making a good body rotation.
In using a one plane method there is very little to go wrong, it is a very simple up one angle and back down at the same angle movement. Mechanically, it is very straight forward with very little to go wrong. The hands and arms do not need to influence the club on the downswing, the movement is made from the body. This type of swing suits players with a more inclined spinal angle at set up and a lower hand position. This set up results in a steeper shoulder incline during the backswing turn and a left forearm position that is flatter at the top of the swing. The hand and arms are very passive during this swing as it is more of a body swing with the power being generated by the body.
However, if you set up with a less inclined spine angle at address and you display a higher hand position, or if you find rotating your upper body more challenging then this type of swing method would cause you problems and you would want to focus on achieving a more traditional swing method, or two plane swing which focuses more on timing and tempo.
Find the correct swing method for your body type and then work on continually making that swing for consistent, accurate and long golf shots.