先后 (xiānhòu) as an adverb means 'in succession, one after another' — it emphasizes that events happened at different times, not simultaneously. It differs from 陆续 (lùxù), which suggests a continuous, often irregular flow, and from 一一 (yīyī), which stresses doing things 'one by one' in a deliberate, complete manner. As a noun, 先后 means 'order, sequence' and is used in phrases like 按先后顺序 (in order of sequence).