Tail swishing is usually a sign of discomfort or unhappiness (ill fitting saddle/tack/ back issues.
It can also be a sign of annoyance or confusion , ie. incorrect or unclear aids.
That said, there are some horses who do it just because they can!
My pony does this when she is cross. If i have asked her to canter round the school she is happy and loves it, if after cantering she is asked to continue schooling in trot she swished he tail.
I had one who did this when he wasn't happy (and it was usually a sign that a full blown strop was imminent). I would say that it isn't a good thing usually...