To stir is to mix something up. You can literally stir cake batter with a spoon, or figuratively stir someone's emotions by writing them a heartfelt letter.
You can stir muffin batter with a wooden spoon, and stir in a cup of blueberries. When people stir, they start to move after being still or asleep: "It was only five o'clock, but he could hear the kids start to stir." And when something stirs you, it brings up some emotion: "That music always stirs up my sad memories." Stir shares a root with the Old Norse styrr, "tumult or disturbance."