萤火

The classic chengyu 囊萤映雪 (náng yíng yìng xuě, 'firefly-bag-on-snow') tells of poor scholars who studied by collecting fireflies in a silk bag (车胤 of the Jin dynasty) and reading by snow's reflected light (孙康) - the firefly is a symbol of perseverance through poverty. Studio Ghibli's 《萤火虫之墓》 (Grave of the Fireflies) uses the same imagery.

萤火 is the light itself; 萤火虫 (yínghuǒchóng) is the firefly insect. Figuratively 萤火 = a small, fragile glimmer of hope or light.