You have an isthmic spondylolisthesis at L5-S1 with isolated disc resorption.See website for details. This condition can be painful but it is really not dangerous. That is- it won’t cause paralysis or put you in a wheelchair but it can cause pain.
You have lost the shock absorption between the spine and the sacrum. If the bone becomes painful from small fractures, this can be debilitating however some individuals have this disorder and function well.
If the spine becomes painful, impact activities will cause the most symptoms. Skiing is an impact activity but you can moderate the amount of impact by staying on groomed runs and using a smooth, graceful style to ski. There are some US Ski team members that have this condition and still race. Only you can determine if you want to ski and take a small to moderate risk of flaring up your back.