So the thing about autism is that it's a spectrum. Depending on if, and subsequently, where you are on said spectrum, the symptoms, support needed, and behaviors change. It's also currently thought to be heavily influenced by genetics. There are several myths connecting vaccines to autism- this is proven false.
While autism is slowly becoming more and more prevalent (1 in every 31 children and 1 in every 45 adults in the US), if you've heard someone say that "everyone is on the autism spectrum", this is completely false. The term "spectrum" in this case is not a straight line/ bar, but rather a dial shape. (for example, negative numbers are not on a speedometer, but they are on a number line (used for visualization purposes only)).
Many common (but not always) symptoms include repetitive/ restrictive behaviors, difficulty with social cues and situations, difficulty maintaining relationships, difficulty multitasking, and/or difficulty adapting to change.
This is just what I learned from a book I read shortly after being diagnosed-- it may be entirely different for others.