This is the 'subject marker / particle'.
ame ga futteimasu.
There is a subtile difference between WA and GA and I don't pretend to try to completely explain it. Years from now, you will still make WA/GA mistakes. Still, in general you can say WA is the main TOPIC and GA is the more specific SUBJECT at hand. In the above example we say it is raining. The topic isn't about rain. We are simply stating the circumstances at the moment and the subject of that particular sentence is rain. If we were to talk all about rain, we would probably start with WA as in:
ame wa sora kara futte kuru mizu desu.
As for rain, it is water that falls from the sky. (You may go on to say more about the overall topic of rain.)
Used with SUKI
watashi wa neko ga suki.
I like cats.
Question words always use GA