On a basic level of bat and ball skills, swing path is a fairly instinctive dynamic of ball striking. It is also a major effector (along with clubface and angle of approach) on ball direction and curve. It can get harder to conceptualise when viewed in relation to swing plane but remember that swing path is only the direction of the path of the club through the ball. It does not reference the club motion anywhere else in the swing.
GC2 Launch Monitor Data
Here is the swing path as depicted by the GC2 launch monitor. As you can see, this player swings from in to out by 9.5 degrees. This is quite a large divergence (from a zero path) in golf terms. Ideally, I would like to see the swing path number no more than 4 degrees either way but you can play quality golf with this path. It just means keeping the face somewhat open through impact so as not to hook it. As you see from the graphic, on this occasion the in-out path has put heavy draw spin on the shot.
Ball Flight Laws
A skilled golfer will be able to adjust their swing path during a practise session based on the curve of the ball and a solid understanding of the ball flight laws. This is definitely worth doing some homework on so you can correctly diagnose any errant ball flights (see ball flights section).
Just as we manipulate the club face in the 'side spin' section, it is well worth seeing how well you can change and control the swing path and what effect that has on the ball.
Due to the combining factors of how we hit the ball from a side-on position and the turning motion of the body through the ball, a lot of club golfers find it hard to conceptualise just how far behind them the club needs to travel from in order to deliver the perfect in-square-in path. Generally speaking, novice golfers get the club out in front of their body too early in the downswing; presumably, in an attempt to have the club moving straight through the ball. It takes many hours of practise to trust that the club will square up from a 'deep' inside path.