"Nous avons sous-estimé l'ignorance des masses."

Simplified Meaning:

People often think that large groups of people know more than they actually do. In reality, many times, big groups can believe things that aren't true or make decisions without really understanding the facts. For example, imagine a group of people spreading a rumor that a certain food causes sickness, even though there's no real evidence. Because so many people believe the rumor, it spreads quickly and causes unnecessary fear. This idea shows that we should not always trust what the majority says or does, just because many people are doing it. Instead, it's important to think for ourselves and do our own research before jumping to conclusions. In our lives, this means being careful about believing everything we hear from others and making decisions based on solid information, not just popular opinion.