100 means that’s when the searches hit their most popular peak while 0 means nothing was really searched. Google trends does not show actual search volumes.
Google Trends looks at search terms compared with all searches done (in a specific geographic region and time point). That data is then normalized.
It’s essentially looking at popularity trends of is this hot or not.
Source: I am a former SEO
Edit: here are specifics about what I mentioned above if you’re curious.
Iowa is just where that term was the most popularly searched. However, it was searched in all states.
I explained Google Trends a bit more here if you’re curious.