The description above is accurate. Here is an illustration of how to gird your loins: http://www.artofmanliness.com/2014/10/02/how-to-gird-up-your-loins-an-illustrated-guide/

'Prepare yourself' would be how I would describe the expression. Understand and be aware of difficulties that may be coming and protect/guard yourself.

It's an idiom. It's an intentionally old-fashioned reference to something soldiers in antiquity did to prepare for battle, so it means "get ready to fight." Because it's so old-fashioned, it's slightly facetious or joking. "It's time for the annual budget meeting again, we'd better gird up our loins." A similar idiom, with a naval reference, is "clear for action" or "clear the decks." In the age of sail, before two ships engaged in combat, they would remove everything movable that was on the deck.

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