It is used to speak in an impersonal way.
Hay que limpiar la casa: Someone must clean the house ( it doesn´t matter who. In English, we would use a passive: the house must be cleaned.
Hay que ser muy inteligente para ser cientifico: One must be very intelligent to be a scientist.
Hay que hacer algo: something must be done.
In next posts we will see some examples of its use, with videos and sound.