After graduating, he set his sights on gaining admission to a top-tier university for his master's degree.

set 在这里是动词，意思是"确定、设定"。 his 是代词 he 的所有格形式，表示"他的"。 sights 是名词 sight 的复数形式，本义是"视线、视野"，引申为"目标、志向"。 on 是介词，表示"关于、针对"。 set one's sights on 是一个固定短语，意思是"把目标定在...上"或"决心要..."，表示某人下定决心要达成某个目标。