Reversibility is the fact that when training stops the adaptations made are lost. Adaptations are generally lost at a similar rate to which they were gained. So if an athlete has put on 10Kg of muscle in 1 month, then gets injured they will lose the muscle very quickly. However, if the athlete as developed that muscle slowly over 6 months or maintained their muscle gain for 6 months then the loss occurs at a slower rate. The more adaptations that have happened the more you have to lose. So elite level athletes tend to lose more than a recreational athlete because they have more to lose. Athletes need to avoid the reversibility that will occur if training ceases. Training can cease for multiple reasons, the two main reasons are: injury and the end of a season. Reversibility can be avoided by maintaining some level of training during the off-season or when injured. This is more problematic for an injured person, but can be achieved by training the uninjured parts of the body while recovering.
For aerobic training the effects of reversibility can be seen 4-6 weeks after training stops. Reversibility can be avoided by maintaining 2 sessions of aerobic training each week.
For resistance training reversibility can normally be seen in 2 weeks. The effects can be avoided by maintaining 1 session a week at the same intensity as previous training.