Otsukaresama desu is a key phrase that you will need to master if you plan to work in Japan. Whever you meet someone who is more senior than you, you say おつかれさまです。
The straight translation of this phrase would be "being tired" ... This means, you are thanking someone for their hard work.
Noe: You do not use this term for someone is not your senior. You say 「おつかれ」「おつかれさま」... Never「おつかれさまです」