Didi and didi mou are Vietnamese phrases that were taken up as U.S. Military slang during the Vietnam war. They mean, essentially, "go!" and "go fast!", and are used quite often by both veterans and current enlistees, probably because they are funner to yell out than are the English equivalents.
In Vietnamese the root word is actually 'ɗi'*. This odd looking character (if you can't see it, it's the letter 'd', but with the upright stroke curved outwards) represents a voiced alveolar implosive. In English we use a voiced alveolar plosive, AKA the letter 'd'. Making it implosive involves closing the glottis and pulling it briefly downwards, something most English speakers can barely imagine. Needless to say, when American soldiers heard "ɗiɗi" they pronounced it as "didi".
Di simply means 'go' in Vietnamese. Repeating it changes it into an imperative, i.e. "Go!" or "You must go". Mou means 'fast' or 'quickly'. (It is often writen 'Mau', which I believe may be the correct Vietnamese. In English it is pronounced to rhyme with 'now').
Didi is often used to mean "run!", and didi mou used to mean "Run!!!". You will also hear them used to mean things like "get away from me!", "get out of here!" or "do it faster!". Usually if you interpret them as if they were the original Vietnamese you will not be far wrong.
* I'm am writing this using the International Phonetic Alphabet; in Vietnamese it is actually written like a lowercase 'd', but crossed like a 't' (đ). The uppercase is written like a 'D' with the crossbar through the straight, upright line (Đ).