A tactical brevity code is a prearranged set of single words or very short phrases which can be used on voice communications channels to convey more complex information in a quick and easily understandable manner. More colloquially, the U.S. military tactical brevity codes are all that cool military lingo that's used on the radio! There are two primary reasons for the use of brevity codes. One, it ensures that two or more parties communicating on the radio don't have to waste time explaining precisely what they mean, especially when time is critical. Two, when using voice communications, any interference will make longer phrasing using common words harder to precisely identify. For example, when there is static on the channel, it is much faster (and clearer) to say "VAMPIRE! VAMPIRE!" than to say "I see incoming missiles, I think they're aimed at one of our ships! No, really! Hey, look out!" This, of course, presupposes that everybody on the channel knows what VAMPIRE means already. Apropos of nothing, the John M. Ford Star Trek novel The Final Reflection had a Klingon 'battle language' which was basically this.
The U.S. military solved this problem the way it usually does - by publishing a field manual. In the case of brevity codes, the manual is publicly available, because the purpose of the brevity code isn't to maintain information security but to allow clear and concise communication. If you're interested, and if you and your friends want to really spice up your online gaming communications, you can read up on the full set of multiservice brevity codes here:
U.S. Army Field Manual FM 101-5-1, Appendix E