An impersonal subject is used primarily in expressions of weather and time. In these expressions the subject doesn't do anything -- it represents, you might say, the way things are- the universe at the present moment. The verb of an impersonal subject is always in the third person.
Il is the impersonal subject of time expressions.
|Quelle heure est-il ? Il est 4 heures.
What time is it? It is 4 o'clock.
Il est midi.
It is midday
WeatherIl is used for weather expressions.
Both ce and il can act as impersonal subjects in conjunction with être and an adjective.
|Il/C' est possible d'améliorer le monde.
It is possible to improve the world.