The herd instinct refers to the thinking that individuals feel they are behaving correctly when they do the same thing as other people. In other words, the more people follow a certain idea, the better or truer we think the idea is. And the more people display a certain behavior the more appropriate this behavior is judged to be by others.
There are any number of examples for the herd instinct, which is sometimes called the social proof. It is when social pressure overturns common sense. It is the herd instinct that makes a teenager follow his friends and take up drugs or start smoking. It is the same instinct that made the followers of Hitler kill millions of Jews. If they had to make a decision by themselves, individually, perhaps they would not have done it.
The herd instinct was very useful to our ancestors who had to survive in the wild. It made sense to follow the group in running away from danger, even if you didn’t see the danger yourself. When your pals began running yelling ‘tiger! tiger!’, you also ran; otherwise you would be dead. Following the herd made sense then, but in the modern world when you are faced with decisions that you need to take individually, some independent thinking is in order.