Einmal, meaning "one time" (a delicious specimen of Denglish used by tourists wearing "Jack Wolfskin" jackets, "basecaps" sandals and socks) is a mean to emphasize the substantive associated with it.
Just like its friend "zweimal", einmal is likely to be heard in situations requiring "perfekt" understanding of what is beeing said. "zweimal Kaffee bitte" would be said in a noisy restaurant, instead of zwei Kaffees bitte (this is my opinion, but there certainly is a more grammatical reason). "könntest du das einmal machen?" is also a very good example of the emphasis implied by einmal. It is sometimes abbreviated and read 'mal.
Also nicht einmal = not even (the emphasis again).
Apart from this "einmal" is also used to translate the English word "once"...
It can also translate the English construction "used to" to describe a distant past :
Ich war einmal ein Kind. = I once was a child.
"Es war einmal eine Prinszessin in einem weissen Schloss, bla bla bla"