When you "retract your scapula," you pull your shoulder blades (scapula) back and down. You can find the position by making backward circles with your shoulders (rolling them backward), then stopping when they are at about a 4 o'clock position, imagining that someone is watching you from your left side and straight up is 12 o'clock.
By retracting (pulling back) your scapula, you put yourself into almost a military "at attention" position, with chest out and up (but don't overdo it).