This is one of the most iconic poses in yoga, and part of the Sun Salutation sequence that's practiced in many classes.
Start by standing on the mat with one foot 3 to 4 feet in front of the other, feet parallel, hips forward. Turn your front foot slightly in toward the center of your body, and your back foot out, so that your toes are pointed at a 45 to 90 degree angle. Bend your front knee and sink down into a lunge, trying to bend as low as possible without letting your knee cross over your ankle. Push your hips forward. While you're doing all of this, raise your arms over your head and reach your fingertips toward the ceiling. Feel yourself stretching tall as you keep your heels grounded and pressing into the mat. You'll want to practice this pose with both legs forward for even stretching on both sides.