In a good golf swing, the backswing and follow through will tend to mirror each other. The path the club swings back on in the backswing matches the path on the follow through. An example is at the top of your backswing, you want to swing the golf club over your right shoulder and at the end of the follow through, the club should be swung over your left shoulder (for the right handed golfer).
The majority of golf instruction has to do with the backswing and the downswing, but the follow through is also important. If you are making a good backswing, downswing, and have a good impact position, your club will tend to get into correct positions in the follow through also.
Golfers who don't have a good swing path with their club on their follow through will probably have problems in the backswing and/or downswing as well. So, working on your follow through can actually help your backswing and dowswing indirectly.
The path the club swings on is kind of a circle. Swinging the club too steeply over your head or neck will lead to problems in the downswing and often result in pulls and slices. Swinging too flat or too much like a baseball swing will cause the opposite problem of pushes and hooks. The checkpoint of trying to swing the club over the tip of your right shoulder on the backswing will really help your swing plane and path so you can deliver the club on a good path in the dowswing and through impact.
To make a good follow through swing path, you want to feel like you are swinging your club head towards your target and then continue swinging it over the tip of your left shoulder to complete your swing. Working on your follow through path will really help to straighten out your golf shots whether you tend to hit the ball right or left.
Another thing that matches in the backswing and follow through is the width of the swing path. However early the right arm bends in the backswing is about where the left arm should bend in the follow through. This will keep the width or arc of the swing path consistent and matching on your backswing and your follow through.
The degree or amount the club head opens up in the backswing should also match and be how much it closes in the follow through. The club should open slightly as the arms rotate in the first part of the backswing and the arms should rotate the same amount on the follow through so that the club face angle matches. Too much rotation of the arms in the backswing leads to an open clubface and not enough leads to a closed clubface. We want just the right amount of rotation to keep the club face square and that amount of rotation should match in your follow through.
Hopefully, thinking about mirroring your backswing and your follow through will help you eliminate some swing faults and straighten out your golf shots. It's easy to think about and can fix a lot of problems related to the swing path or swing plane.