How can autism be cured and what is its cause what does it mean to be on the autism spectrum by definition, arent we all on the spectrum?

There is no cure for ASD (autism spectrum disorder). We are not all "on the spectrum" by definition, but certainly it is a spectrum with some people functioning differently than others. Here is a good summary: https://www.cdc.gov/ncbddd/autism/facts.html

Decades ago autism was a single issue but similar features were seen in other conditions like Asperger Syndrome and various forms of unclassified developmental delay. Educators & treatment teams proposed unifying the issues under a single umbrella term or spectrum to help understand what treatment/educational approach could help any or all of those affected by the disorders.