I think the party went well - people seemed to be enjoying themselves.

enjoying 是动词 enjoy 的现在分词形式，enjoy 的意思是"享受、喜欢"。 themselves 是反身代词，指代前面提到的 people（人们），表示"他们自己"。 enjoying themselves 是一个固定搭配，意思是"玩得开心、自得其乐"。