It's technically an Army term, which Wikipedia defines as "a battle cry... referring to or meaning anything and everything except no." Some popular usages of Hooah include:
Heard, understood, and acknowledged
What to say when at a loss for words
Roger, good copy
Glad to meet you or Welcome
Did not hear what was said, but not going to ask to repeat.
However, a new-to-the-army wife (at the time, anyway) and very close friend of mine, when seeing this word on a random Facebook post of mine, was confused by the term and thought I was perhaps giving the phonetic spelling of "whore" with a Jersey-ish accent. Hooah. We thought this misunderstanding was hilarious, and henceforth have added