Take one feature, being attention to detail, or being able to filter out detail and gloss over things. (Seeing the trees or the forest.)
- Some people go through life blissfully ignorant of the detail in the world around them. Even though I think they'd be far from where autism starts, I wouldn't call these people normal, but it's still part of the spectrum of human attention to detail. (Always see forest, can't see trees.)
- Some can see trees when they're needed, and forest when it's needed. This would be normal.
- Some people can put their natural attention to detail it to good use e.g. by cataloguing library books. I also wouldn't call librarians normal . There are quite a number of professions that benefit from some mild to moderate autistic traits. (More inclined to see trees, but are able to see forest.)
- Other people get so overloaded by the amount of detail around them, and treat every detail as being of equal and vital importance, that it's all too much to cope with and they can't function properly because of it. (Can't see forest. Even trees might be too general. Might see each vein on each leaf and each tiny bit of bark.)
And every possible gradation between and beyond those examples make up the attention to detail spectrum.
Now couple that with other features of autism, which are also ranged on their own spectra and you can build a picture of the autism spectrum.
If someone with proper autism knowledge thinks my simplified explanation is plain wrong let me know and I'll remove it. I'm just an amateur in the topic.