亮亮的 is an ABB-style reduplicated adjective (here AA
的) — doubling
亮 (bright) and adding
的 softens and intensifies into a vivid, descriptive 'brightly-shiny'. Very common in Chinese to add liveliness:
红红的脸 (rosy face),
圆圆的眼睛 (round eyes),
香香的菜 (fragrant food). Note: tone of the second
亮 is often pronounced as a light first-tone (liāng) in some regions.
