Activists were handing out leaflets about environmental protection in the city square.

leaflets 是 leaflet 的复数形式。leaflet 由 leaf（叶子）和后缀 -let（表示"小"）组成，字面意思是"小叶子"，引申为"传单、小册子"，因为传单的形状像小树叶一样轻薄。