This is another easy fallacy and is also known as the argument from popularity or an appeal to the masses (among several other names). When you were growing up, you might have told your mother you wanted to do something because all the other kids were doing it. Her response probably included something about jumping off a bridge. Your mother saw right through your argumentum ad populum if that was the case.
This fallacy uses popularity as an argument for accuracy or relevance. Sometimes, an action or thought may be correct if lots of people agree. However, the majority of people could be very wrong about something and not realize it.
I get told all the time that Christianity must be true because so many people believe it or, more generally, that a god must exist since so many people believe in him. However, popularity is poor evidence for the truth of something. The majority of people once thought it perfectly fine to keep slaves. It was once the majority opinion that women were inferior and had to stay at home and pop out the next generation. It was once considered a swell idea to burn people at the stake or imprison them for blasphemy. Did that make any of these actions correct? Did it make them just and right and good?
I see this fallacy as a kind of grown up peer pressure. We always hear about teens facing peer pressure, but the need to “fit in” is just as strong with adults. We are a social species. The argument from popularity appeals to the part of our brain that wants to belong, to go along with the masses so we don’t stand out.