Strength exercises help you build and maintain muscle mass. Short term, the extra muscle mass means you will burn more calories (even at rest). Longer term, maintaining healthy muscles is the best way to combat the effects of aging.
Do 2-4 strength exercises every week. It doesn’t matter how many reps you can do or how much weight you can press as long as you push yourself. But don’t overdo it either. Learn to tell the difference between a healthy muscle ache and pains that signal you should take it easier.
For our cheat sheet users, these exercises are considered strength exercises (engage in the exercise for at least the indicated number of minutes):