"Indexing: Google Trends data is pulled from a random, unbiased sample of Google searches, which means we don’t have exact numbers for any terms or topics. In order to give a value to terms, we index data from 1-100, where 100 is the maximum search interest for the time and location selected."
"Normalization: When we look at search interest in a topic or query, we are not looking at the total number of searches. Instead, we look at the percentage of searches for that topic, as a proportion of all searches at that time and location."
from https://newsinitiative.withgoogle.com/resources/trainings/basics-of-google-trends/ and see also https://support.google.com/trends/answer/4365533