Aikido has a vast curriculum from Taijutsu (体術, literally "body technique" or "body skill") to weapons ( Jo and bokuto) training. Ukemi or rolls and fall, are the most fundamental techniques that have to be learned.
For some teachers, Ukemi means the body: mi that sacrifices: Uke for the perfect execution of the technique which is performed by nage. A more gentle meaning is Uke is the one receiving the technique and accepting the fall without any resistance or confrontation and helping the nage to better perform the action. This is part of the overall philosophy of Aikido in general and Kobayashi-Ryu. We do not oppose.
A beginner student will start to learn the first two fundamental rolls: Mae-Ukemi and Ushiro-Ukemi.
Mae-Ukemi is a forward roll.
Ushiro-Ukemi is a backward roll.
As the student becomes more comfortable, more advanced ukemi will be taught such as sutemi or breakfall
It is the responsibility of the advanced students (Senpai) to protect the new students (Kohai).