This is the 3rd most frequent Japanese word.
が
"が" is a Japanese particle marking the subject of a sentence.
The particle 'が' marks the subject (猫) of the predicate expressing the speaker's preference.
私は猫が好きです。
I like cats.
Here, 'が' is used to mark the subject (空) of the descriptive sentence.
空が青いです。
The sky is blue.
In this sentence, 'が' identifies the subject (雨) of the action occurring.