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The original idea is the stereotype of autistic children who start screeching when something happens that they don't want.

The underlying phenomenon is people failing to cope with discomfort induced by things others don't perceive as uncomfortable. I'm not a psychologist, this is more an informal way to express my own experiences.

For example, a person (possibly, but not necessarily autistic) with sensory issues may find grocery store visits unbearable if the lighting of that store triggers that discomfort, or the hum of the AC, or the general noise of the place. Trying to ignore this discomfort may work for a while, trying to block the triggers may help, focusing on some reward or comfort may help the brain hold on to more pleasant thoughts, but if those fail, eventually the brain will reach a point of overload.

It's particularly bad when some method of coping suddenly fails or the discomfort spikes suddenly. If I was trying to make it through a grocery trip by promising myself some comfort food, then reach the shelf where that food is supposed to be and find it empty, that cuts quite a gash into my mental barrier. Some noises, when unexpected and loud enough, can bring me all the way from "calm" to "overload" in an instant

The exact reaction can differ, with some options being less visible. I usually enter a type of dissociation where only the most routine things still work (like beelining for the checkout, paying and getting the fuck out of there, rest of the grocery list be damned), but since I still function to some degree, it might not look like an overload.

The most visible reaction is probably when the "dam" trying to contain the discomfort breaks so violently that it turns into acute pain, with the result looking much like you'd expect someone in acute pain to react: screaming.

Thus, the most noticeable expression of Autism in people you don't know is probably when something seemingly minor sends them over that edge. You don't see the buildup, you don't see the other ways people deal with the discomfort, you might not even understand the discomfort itself, but you see a child suddenly breaking down and screeching.

There are other things too, like breaking from established patterns, but this is getting top long already. The mechanisms is similar, in any case: The break from the pattern produces discomfort, a sudden break producing sudden discomfort, which can lead to the same kind of overload.