Senpai (せんぱい) means your senior: someone who joined your school, club or workplace before you. You know the meme — but in Japan, せんぱい is not a romance trope. It's an everyday word with real rules, used in every school, club and office in the country.
What it actually means
Your せんぱい is anyone who entered your school, club or company BEFORE you. Age doesn't matter — entry order does. A 19-year-old can be the せんぱい of a 40-year-old who just joined. The opposite is こうはい (junior).
You can attach it to a name, or use it alone like a title. One asymmetry to remember: nobody is ever CALLED こうはい to their face — it only describes the relationship.
Being older is not enough. Someone is your せんぱい only inside a group you share: the same school, club, team or company. An older friend you met somewhere else is simply a friend, while a graduate of your school stays your せんぱい for life.
せんぱい, せんせい and こうはい
Learners often mix up the first two. せんぱい is a fellow student or colleague who is a step ahead of you; せんせい is your teacher, or your doctor. Both begin with the same kanji, 先, “ahead”. The person a step behind you is your こうはい.
Why anime is soaked in it
School clubs are the senpai/kouhai system in miniature — respect, admiration, unrequited feelings across a one-year gap. That's why the word carries so much drama: it names a relationship, not just a person.
ふじせんぱい、 おしえてください！
Fuji-senpai, please teach me!
まかせて！
Leave it to me!
Can you use it?
Absolutely — at a Japanese school or workplace it's alive and well, and it's a graceful way to avoid saying “you”: use name + せんぱい, or just せんぱい. One honest note: nobody says “notice me, senpai” in Japanese. The catchphrase is an English-speaking-fandom invention.