In golf, flaring one’s foot corresponds to angling it out slightly so that the line of the foot is not perpendicular to the target line. This is in contrast to seeing both feet parallel to each other and perpendicular to the target line. You can decide to position your feet likewise or flare either of your feet. The pros and cons of such tweaks are explained.
In addition to how they are positioned relative to the ball, your feet can also vary in how they are positioned relative to each other and specifically the distance between them. Indeed, they can be placed very near one another in a narrow stance or they can be placed far from each other in a wide stance. Choosing which stance applies to what circumstances is detailed.