This is the 559th most frequent Italian word.
libera
"Libera" in Italian means "free" in English.
Here, 'libera' is used as an adjective describing 'air', implying the quality of being free, unrestrained.
L'aria di montagna è sempre così libera e fresca.
Mountain air is always so free and fresh.
In this sentence, 'libera' describes the speaker's feeling, referring to being unburdened or relaxed after work.
Dopo una lunga giornata di lavoro, finalmente mi sento libera.
After a long workday, I finally feel free.
Here, 'libera' indicates that the chair is not occupied, thus available for someone to use.