Progression is one of the essential factors if you want to build muscle and strength effectively. If you increase the training stimulus by increasing training volume, fatigue will accumulate within a period. This fatigue negatively affects your performance in training.
The performance of your central nervous system is reduced. Furthermore, your body reacts to the increasing fatigue with hormonal adaptations. These adaptations include the production of endogenous growth hormones. Production continues to cease as fatigue level increases. This negatively affects muscle growth.
So, in the long run, it becomes more challenging to progress in strength training. If you disregard the signals your body gives you, it will be almost impossible to achieve significant performance increases at some point. Your body will not be able to recover fully.
To reduce training exhaustion and maintain the potential of your training, you should incorporate phased training sessions with lower training volume and intensity into your plan. These phases are called deloads. Usually, a deload is performed for a week.
Deloads are part of active regeneration. It means that you perform light physical activities (low-intensity training) that promote your regeneration. If you rest completely without stressing your body, this is called passive regeneration.
During a deload, you significantly reduce the sets and the weight. For once, you should not train as close to muscle failure as usual during this phase.
You don't have to worry about your muscle mass during a deload. It will be preserved. It is much easier to maintain muscle mass than to build it up.
After a deload, fatigue should be gone entirely in the best case, so the next training cycle starts with full energy. Training volume can be adjusted or increased again.
Training beginners do not necessarily need a deload initially because they usually do not have a high training volume. However, if signs of high fatigue become apparent, recovery does not keep up, and progression can not be achieved anymore, a deload should be considered.