This is the 1457th most frequent Japanese word.
空
"空" (sora): sky; emptiness; void.
The word '空' (sora) refers to the 'sky' in this sentence.
空は青くて美しいです。
The sky is blue and beautiful.
Here, '空' (kara) is used to mean 'empty' or 'vacant' space.
その部屋は空になっています。
That room is empty.
'空いた' (a ita) derived from '空く', represents 'available' or 'vacant' time.