The standard Romanized Klingon spelling is Qapla', and as ƃʍɐp ʇıdsuɹnʇ mentions, it is a noun meaning "success" (or sometimes "absence of failure").
It doesn't encode any information about person, but is never used in anything but the third person.
The number is not specified, but it's usually used to speak of success as a concept, rather than of individual successes. I've never seen anyone use the plural, which would be Qapla'mey.
It can be used in sentences, but is often used as a sentence by itself, and is then taken to be an expression of a wish for success (or non-failure) for oneself or for others, or to congratulate another on their success.
Klingon #1: romuluSnganpu' vISuvrup. - "I am prepared to fight the Romulans."
Klingon #2: Qapla'. - "Success." - "May you succeed!"
Klingon #1: romuluSnganpu' vIHoHta'. - "I have killed the Romulans."
Klingon #2: Qapla'. - "Success." - "Well done!"
A related word is the verb Qap, meaning "to success", "to function", "to be in operation" or "to win".
bIQapjaj. - "May you succeed!"