This isn't a video from our style of aikido, but it is very smooth, very relaxed practice.
One of the things everyone practices is rolling and falls. To be able to train with the other members of the class, it is important to be able to effectively take falls in all directions. In aikido, the person attacking is the uke. The person receiving the attack is the nage. Having committed to the attack, the uke will have to take the technique that the nage performs. This is called taking ukemi.