You will see in the Aikido lexicon the term hamni. Hamni is the fundamental stance in Aikido. Hamni is performed by placing yourself in a T-stance, feet shoulder width or less apart, balance down the center of the body in both directions. Your knees should be bent and your back foot should be driven back so that the weight is primarily on the knife edge of the foot. Your belt knot should be facing forward. You should have a solid kokyu in your arms.
If your left foot is forward you are in a left hamni. If your right foot is forward you are in a right hamni.
Many of the exercises and techniques in Aikido have the Uke and Nage in either the same hamni (both right or both left) or opposite hamni (one right and one left). You will find in class and in online/written materials that these paired hamni stances go by either the name “Ai Hamni” or “Gyanku Hamni”. (Sometimes without spaces “aihamni” and “gyankuhamni”.)
When you are asked to change from one foot forward to another foot forward hamni you should make sure to keep your stance solid. Do not “pop out” of your stance and stand up. Instead keep your head level, keep kokyu in your arms. Keep your focus.
Sensei teaches that Aihamni is when Uke and Nage are in differing hamnis (one left, one right). When Uke and Nage have their centers matched this results in the forward foot pointing at each other. Uke and Nage look at each other it is like “they are looking in a mirror”.
Of course Gyankuhamni is the opposite of Aihamni. Sensei teaches that Gyankuhamni occurs when the Uke and Nage have opposite feet from a “mirror” perspective. In other words both facing each other standing in the same hamni.