A lightstick was a device that emitted light.
In 2375, Mila used lightsticks to provide light in the basement of Enabran Tain's house that Kira Nerys, Damar and Garak were sheltering in. Cardassia Prime was experiencing a blackout at the time. (DS9 episode: "What You Leave Behind")
In 2376, Kira uses lightsticks when she, Kasidy Yates, Ro Laren and Yevir Linjarin visited B'hala. (DS9 novel: Avatar, Book Two)