Improving power output, in regards to fitness, is about how quickly we can produce force. We see expressions of power all the time: a fast, crisp deadlift from the floor, jumping to a box, side tossing a medicine ball and shouldering a d-ball all express power.
As we age, power output, not strength is the first thing to decline. We want to keep the ability to move quickly and rapidly, producing force over time. It will help us keep power up and aid in maintaining strength.
Put a little extra pepper on those medicine ball slams or shotputs. Try to move the bar quickly when standing up from a squat and be explosive on those swings. We can do dedicated power training, things like olympic lift variations or dynamic effort loading protocols, however, we can also try to improve our power output by being deliberate during strength work. Move that bar!