While waiting in the mulberry trees, King David and his troops awaits the sound of God
moving through the trees before advancing upon the hapless Philistines.
Image from the Jim Bakker Show Blog
2 Samuel, Chapter 5 - This chapter is about King David rousing his troops and the subsequent assaults and victories on the Jebusites and the Philistines, along with some of the repression done by the Israelites to keep these people in line.
It starts off with some grumbling from the Jebusites, who are not a fan of David and don't want him around. So, David takes the Jebusite stronghold of Zion, which becomes the City of David. He rallies the people to his cause, saying in the eighth verse: "Whoever gets out of the gutter and kills the Jebusites, and the lame and blind whom I hate, that person shall be made a captain (in the army)."
As King David established his kingdom and it grew, David gathers more wives and concubines. In other words, David has his own harem! It doesn't say how many, but they started getting pregnant real quick.
Word of King David's rising kingdom came to the nearby Philistines, who decided to seek him out. It isn't clear in the King James version if they were seeking him out for military means, or just to meet the new king. But what happens is that after the Philistines come and camp in the nearby valley of Rephaim, David inquires in Yahweh (aka God) what he should do. Yahweh, of course, wants David to slaughter all the Philistines. God and David both destroy the Philistines in the valley of Rephaim.
However, more Philistines came and once again camped in the valley. So, God lets David in on a bit of strategy. He advises David and his troops to go around the Philistine encampment into a grove of mulberry trees, there to await "the sound of a going" in the tops of the mulberry trees (5:23). David follows the advice and waits patiently until indeed a rustling was heard in the trees. He gave the orders to advance upon the Philistines and left death in his wake.