AKA Side lunge
The lateral lunge is a unilateral leg exercise moving in the frontal instead of saggital plane, emphasizing hip and adductor strength and stability.
With a barbell on your back, take a very wide stance with the toes turned out. Keeping your trunk as upright as possible (it will incline forward to a similar degree as it will in a back squat), shift to one leg as you bend at the knee and hip like a squat, keeping the other leg straight and foot planted. Stand to the starting position and continue into a lunge on the other side.
The lateral lunge is a good prehab and stability exercise that exposes the hips and legs to a very different stimulus than typical weightlifting exercises that tend to be both bilateral and limited to the saggital plane.
Sets of 5-10 reps per leg are appropriate, but higher reps can be used, particularly in a rehab situation or when first introducing the exercise with very light weights to prepare an athlete for more.