。
Her heart was filled with joy.
Literal
She [topic-は] joy [from-で] chest [subject-が] became-full.
胸がいっぱいになる ('one's chest becomes full') is a body-part idiom for being overcome with emotion — most often joy, gratitude, or bittersweet feeling. 胸 ('chest') in Japanese serves the metaphorical role English assigns to 'heart': it's where emotion lives. The で on 喜び marks the cause that fills the chest. The phrase is heard frequently in Japanese acceptance speeches, weddings, and emotional moments — a polite, self-effacing way to say 'I'm overwhelmed.'