In Spanish there are different types of impersonal sentences, i.e those that do not have a subject, generally because it is not relevant or we do not know.
Using the 3rd person singular of the verb
1. Referring to the weather
It snowed yesterday.
Amanece muy temprano.
It gets light very early.
We cannot extract any type of subject from these sentences. ¿Who snowed? I don't know.
2. Using the verbs haber, hacer and ser
Hace un día estupendo.
It's a lovely day.
Había atasco en la carretera.
There was congestion on the roads.
Es muy tarde.
It's very late.
3. Using "se" with the 3rd person singular
Se duerme bien en esta cama.
One sleeps well in this bed.
Se vende piso en la playa.
Flat for sale at the beach.*
*This last sentence can also be used in the 3rd person plural if the object is plural:
Flats for sale at the beach.
Using the 3rd person plural of the verb
Using the impersonal 3rd person plural
Dicen que va a hacer frío mañana.
Apparently it will be cold tomorrow.
Llaman a la puerta.
Someone [we don't know who] is knocking at the door/there's someone at the door.