Interest over time in Google Trends just means searches over time. Google calls it “interest over time” rather than “searches over time” because technically they don’t display raw search volume numbers on the y-axis. Instead they show searches on a scale of 0-100.
To see actual search volume data on the y-axis, install the Glimpse plugin here, which enhances the default Google Trends experience.
How accurate are Google Trends’ Search Interest Graphs?
Google Trends is widely considered to be highly accurate and reliable. It's commonly cited by top journalists and publications like the Economist, the New York Times, and the Wall Street Journal. Google Trends also uses takes several extra steps to ensure the accuracy of the data, for example ignoring repeat searches from a single user over a brief period of time.