To explain a deload one must first understand the concept of training.
What is training? It is the mechanism by which we induce stress on the body resulting in the required adaptation that training stress is targeting.
There are many adaptations we aim for in the process of training from strength increases to muscle hypertrophy and aerobic conditioning. Sometimes training can target multiple adaptations, which is very normal.
The body itself at its fundamental level is a survival organism. It does what it has to to survive long enough to procreate and prolong the existence of our species.
We use this theory to our advantage by using training to place stress on the body, forcing the body to adapt and survive.
When we train the body adapts based on the training stress applied. High stress can result in big adaptations but it's coupled with high fatigue levels. Lower stress is the opposite - low fatigue and smaller adaptations.
So based on that it's easy right? Just destroy someone with high training stress non stop. WRONG! Eventually that accumulation of fatigue will catch up and those adaptations will reverse and you'll be wondering why you're getting weaker or you are not getting bigger. Quite often as well that massive amount of fatigue means a sharp reduction in movement quality which can lead to injury. Not really conjusive to long term progress.
This is where the deload comes in. It is a period introduced in training (usually a week) where intensities and training loads are shed away to allow the body to recover in order to then continue the training stress.
Quite often you'll see an elite or sub-elite lifter simply take a week off training or their training loads reduced to about 50%. This is normal and productive. Recovery is quite often underrated in this game but take it seriously and apply deloading in to your training where appropriate.
It is possible to avoid big deload periods by undulating training stress, which a smart coach will do. We need periods of high stress and periods of low stress, both equally as important as the other!