Hammi refers to a half stance or a step. Hammi is the same as walking along and stopping when the foot strikes the floor. The front foot points slightly to the outside. The heel of the back foot is about on a line drawn through the front foot and is less than a 90 degree angle from the front foot. It is very important that the angle between the two feet is not greater than 90 degrees because then nage is unstable when pushed from the front. Experts opinions range from both feet toward Uke for more power to a 90 degree angle for more stability. I doubt that it is very important. The feet are about shoulder width apart.
When Uke and Nage are in the same hammi, both have the same foot forward, it is called Ai-hammi. If Uke and Nage are in the opposite hammi, one has the left foot forward and the other has the right foot forward it is called gyaku hammi. Properly the spelling is changed to gamae. It would then be gyaku gamae. Left hammi is hidari gamae and right hammi is migi gamae.
We never really talk about stances and we do not have them in the sense that most martial arts use stances but we actually do have stances and they are very important. When we cut down in a throw or strike with a sword we are in a wide strong posture which karate people would call a stance. When we finish the pivot in tenkan, finish the step back in tenshim, or finish the main step in irimi we are in a wide powerful posture that karate people would consider a stance. Stances allow tremendous power but they restrict mobility. Mobility is a major issue in Aikido so we never stay in a stance unless we are demonstrating the finish of a technique. Even then it is not martial art. We are careful not to drive our partners into the mat because we want them to live. In real combat you do not want your attacker to jump up and attack again. Practice is a problem because you cannot practice the real finish of a throw. For this reason it is important for martial training to exaggerate the feeling of a powerful finish so it will be there in real life. Often we get lazy in practice and stop the technique when our partner has lost balance and starts falling. This looks casual and cool but it is not martial training. The point of this is that we normally stand as in walking where we are very mobile and look very casual. When we are projecting force we pass through wide powerful positions that allow projecting considerable force but we do not hold these positions.
Many tyros tend to waddle around on the mat and not stay in hammi. Every move in Aikido is important and precise strong steps that stay in hammi or a wider stance are probably the most important part of Aikido. All power in Aikido comes from body movement and stepping and stance are vital parts of body movement. Never just shuffle around to get to the correct place in Aikido.