放大 means 'to enlarge, to magnify' and is used for making something visually or audibly bigger (e.g., a photo, text, sound). It differs from 扩大 (kuò dà, 'to expand'), which is used for abstract scope, scale, or influence (e.g., business, territory). 放大 focuses on physical magnification, while 扩大 focuses on expansion in range or extent.