Herd behaviour refers to the behaviour of individuals (or animals like schools of fish, flocks of birds and mobs of kangaroos) within the group.
The term comes from ethology (the study of animal behaviour) is often used within the context of people (or animals) behaving without any clear origin of direction or coordination. Typical examples are the sudden movement of the herd in response to the threat of predators or the stampede of investors all trying to leave the stock market at the same time to limit their losses.
In both cases, the behaviour is a response to fear and is guided by what other individuals within the group do. Hence, individual behaviour is characterised by similar behaviour across of mass of individuals and copied behaviour.
Besides ethology, similar concepts (e.g. conformity, bandwagoning, information cascade, groupthink, peer pressure, keeping-up-with-the-Jonese, etc) have been referenced in sociology, social psychology, epidemiology, marketing/consumer behaviour and (behavioural) economics and (behavioural) finance. This makes herd behaviour a common feature of individual human behaviour.