Mean can mean you are being rude or in math terms it is adding a set of numbers then dividing by the total amount. ex. If your numbers are 1,2,and 3 add them to get 6 and divide by 3. You get 2.
It looks like someone has misremembered the words that Neil Armstrong first spoke on the surface of the moon: "That's one small step for (a) man, one giant leap for mankind".