The head is the foam at the top of the beer. It is generally accepted that the lifetime for the head is for it to stay on top of the beer for at least three minutes before it begins to disappear.
The occurrence of the head is mainly determined by small irregularities in the glass which cause the carbon dioxide bubbles to form. The composition of the beer also plays a role: the more proteins there are in the beer, the smaller and firmer the bubbles, and vice-versa. The lifetime of the head says something about the quality and flavour of the beer, but has no effect on the health of the consumer.