Hebrews 11:1 offers a definition of faith. "Now faith is the assurance of things hoped for, the conviction of things not seen." (Hebrews 11:1 ESV)
A very freshly inaugurated King David, in 2 Samuel 5:17-25, has an army of Philistines to go up against. David knows they have to be dealt with so he prepares himself to deal with them by defense and offense. By placing himself at the stronghold, he's ready to go out and take them on, or defend himself as needed. While in the stronghold he calls out to God to determine what to do and God responds to him telling him to go up against the Philistines and take them on. The bible says God's words exactly were "Go up, for I will certainly give the Philistines into your hand." David has a choice here. To obey and take on the Philistine army, or doubt and stay within the stronghold. Knowing David's history, it may not come as a surprise that even if the numbers aren't in his favor, he choses to obey rather than doubt, to head out and attack. This is the essence of faith. David didn't know how it would work, he only knew that God was going to make this happen somehow, and more than that, he took the time to ask beforehand. But David was smarter than I am in that he consulted the Lord first. It is not uncommon for me to plan and strategize and seek good counsel, only to jump out in action without ever consulting the Lord.
After this victory, the Philistines come up again against David in the same area. Again David stops, and inquires of the Lord if he should go up against this next army before he makes any moves. Now, if it were me, I'd probably think to myself, “This has happened before, I know how to do this, it will be just like last time.” But again, David is wiser than I am and inquires of the Lord before acting. This time the Lord has a different strategy, and his answer to David is don't go up against the Philistines. He has a different plan. He outlines a strategy that wouldn't make sense to anyone, but the instructions are clear. David is supposed to circle around behind their armies, without the safety and security of the stronghold. He is supposed to wait there until he hears “the sound of marching in the tops of the balsam trees”. At that point, David is supposed to go up against the Philistines. We don't know what God does or the method He uses to strike down the army of the philistines, but David holds his position and waits. The signal comes just as God said it would, and David heads out on cue, and again, he sees victory.
These victories came about, not because David was a brilliant strategist, though scripture shows that he was. They didn't come because David was a man of action, though he was that as well. These victories came in both cases because God acted on behalf of His people, and David's obedience is an act of faith. The author of Hebrews in 11:32-34 affirms this faith that David had in the Lord, through which he saw the conquering of kingdoms and obtaining of promises. It would have been easy to go out and attack without consulting the Lord. It would have been easy to stay in the stronghold and avoid the battle. It would have been easy the second time around to just go out and fight without consulting the Lord. It would have been easy to head out in fear rather than wait for the sign from God.
Faith means believing in what we are told from God and acting accordingly, rather than what makes the most sense to us. It means obedience in the face of not understanding what is going on whether it is just the circumstances or contrary to logic. Trusting God, and acting on it.