This is the 10803rd most frequent Korean word.
그녀가
"She" or "her" (subject/topic marker).
Here, 그녀가 is the subject and refers to 'she', with the topic marker emphasizing her role in the action.
그녀가 웃으면 세상이 밝아져요.
When she smiles, the world brightens up.
In this case, 그녀가 again acts as the subject, emphasizing the person initiating the action.
그녀가 이야기를 시작했을 때 모든 사람들이 경청했어요.
When she started speaking, everyone listened attentively.
Here, 그녀가 is used to indicate the specific subject (her) of the subordinate clause.