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.