subject marker が (new info)

The particle が marking new, previously unestablished information — contrasting with は, which marks a topic already on stage. Common in scene-setting and event-introducing sentences, and in answers picking out 'X is the one that...' (identificational focus). 猫がいる ('there's a cat' — new info) vs 猫はいる ('as for cats, there is one' — topic already in play).