"It is quiet in the forest."
Translation:В лесу тихо.
This a rudiment of an old locative declension form. For most nouns, their locative form became identical to the prepositional form, so we usually do not distinguish them. Yet you can find a handful of old Russian words, for which the two forms are still different. With some of then, the locative form is disappearing as we speak. For example, you can still say "в дому", but "в доме" is equally acceptable. "В лесу", however, is the only acceptable form.
тихо в лесу
As others indicated, there is a [cutely obscene, if there is such a thing] song that schoolboys in Russia thing, in which each verse starts with the above line. E.g.,
тихо в лесу, только не спит барсук. знает барсук, что его [...] повесят на сук, вот и не спит барсук.
тихо в лесу, только не спит енот. знает енот, что его [...] рот, вот и не спит енот.