It refers to the maximum number of people that a topic is relevant to. It tells us what portion of the population currently engages in the topic or the trend one has searched for (includes those who create and drive the conversation, those who follow and engage more passively, and those who are sucked in by similar conversations).
It's the maximum size of the culture you're studying.
MotivBase Trends offers two population sizes:
current: within the time period of the search
predicted: after 1-5+ years, depending on the search terms