If you want to be technical, it goes like this:
uu_u_u_ _//uu_uuuu x.
To put it in another way, as Oxford Living Dictionaries tells us, galliambics means relating to or consisting of two catalectic iambic dimeters...*
...no, no, come back!
Okay, if you don't want to be technical, then it's a rhythm thing, and this superb video, uploaded by fiatlapides, shows you everything you need to know (it starts in Latin, but there's an English version about six minutes in):
Do galliambics have any relevance at all for those of us who seldom speak Latin?
Not really, though Tennyson imitated the form in Boadicea - but that doesn't stop it being simply terrific, does it?
Thing To Consider Today: galliambics. This word was made up in the middle of the 1800s from the Latin galliambus, a song of the Galli. The Galli were the priests of Cybele. And often, as I said, frenzied.
*In other words, according to Collins Dictionaries, four lesser Ionics.