In the general meaning, “Senpai” is a person who is older than you are, for example Seniors in school. We often use this word at school and work place calling the person who lived in that place before us.
[Example sentence] Kinou, Koukou-jidai no Senpai ni Atta. 昨日、高校時代の先輩に会った。 きのう、こうこうじだいのせんぱいにあった。 I met my senior from high school yesterday.
Kinou: yesterday Koukou-jidai: from high school Senpai: my senior Atta: I met