Definition
(used imitatively) A cliché used to exaggerate an accomplishment or milestone.
Origins
Spoken by Neil Armstrong when he first stepped onto the surface of the Moon. Armstrong intended to say "one small step for a man", but misspoke.http://www.snopes.com/quotes/onesmall.asp
How People Actually Use It
A famous quote said by Neil Armstrong when he set foot on the moon
"During his flight to the moon in an Apollo 11 rocket, Neil Armstrong stated, “That’s one small step for man, one giant leap for mankind.”"
Source: Urban Dictionary