 Being overstimulated for an autistic person means that they are experiencing sensory overload. Now, it isn't- Not always. It can be a lot of different things, it can just be general stress, it can be due to social difficulties, but sensory overload is definitely like a contributing factor to having like a meltdown, a shutdown, being overloaded, 100%. It's important to note that sensory overload isn't just exclusive to autistic people. However, it is seen in them the most often and causes more problems for them, as it takes less to set it off than someone who isn't autistic. Things that may cause sensory overload are lighting, smells, sounds, certain out-of-place or repetitive motions, and textures. It may also be as simple as an overwhelming situation, where a lot of negative things are happening one after another, or where there's a change from a routine that they aren't expecting. If they- The routine change is definitely a big thing.