This fallacy hinges on the belief that if everyone is doing it/ believes it then it must be true. Think of lemmings. One jumps over a cliff and more follow. Many of our moms would say "Well, if Johnny jumped off a bridge would you do it too? I don't care what Johnny is doing."
There have even been a few social experiments that have proven that people will believe things just because others do. Telling people 74% of Americans believe XYZ can make some people believe it as well. For example, if I tell people that 84% of Americans wear green on Tuesdays, people will begin to believe that wearing green on Tuesday should be a thing. I mean 84% of people think it's okay. It must be. This is a simple belief that will not affect much other than laundry, but there are debates that can be much more significant.
Most of these revolve around belief in a deity or religion. "You can't tell me that 3/4 of the world's population is wrong. There is a higher power." Now, I am not actually going to debate whether or not God or a god exists. What I am saying here is that God doesn't exist simply because people agree that He does. To further that, I am not going to say that He doesn't exist simply because large numbers of people say that he doesn't exist. The world isn't round because lots of people say it is. The world is round because it can be scientifically proven to be round. I don't care how many flat earthers say that the world is flat. Their agreement doesn't mean that it is true.
Also, be careful about jumping on a bandwagon. Slavery was popular amongst Southern plantation owners in 1785, but that doesn't make it right. It was still wrong and harmed thousands of Africans and African Americans.