Sensory burnout describes a specific pattern: the system reaches overload through the routes that process incoming information from the environment. Light, sound, texture, temperature, smell, the physical presence of other people. These routes have a cost. When that cost exceeds what the system can currently afford, the system begins to close them.
This is not the same as disliking sensory input. Most people, in most states, tolerate or actively enjoy many kinds of sensory experience. Sensory burnout is what happens when the cumulative load from those routes exceeds the available threshold, particularly in a system that processes sensory input with greater intensity than average.
The autistic connection
Autistic people frequently describe sensory processing differently from non-autistic people. Input that others filter automatically may reach the system at full volume, requiring active processing where others require none. This is not a problem to be fixed. It is a different architecture. But it does mean the threshold is reached earlier, under less dramatic-looking conditions, and the burnout that follows can be deeper and slower to resolve.
This is one of the reasons autistic burnout is often misread. From the outside, the conditions do not look extreme. From the inside, the system has been processing at a level of intensity the outside cannot see.
What does not help
Telling someone in sensory burnout that the environment is not that bad is not a useful intervention. The cost is not a matter of opinion. The system is doing exactly what it should: protecting itself from overload it cannot currently absorb. What helps is reduction of load, increase of recovery time, and removal of the prediction that overload is inevitable.