Introduction to the Guide.
Sequences are differences in position of an object. Objects can be anything, but in this case, it's just a grey circle. It could be something as complicated as a giraffe, or could contain gaffes, such as intricate text in bubbles that surround it.
Objects within themselves can be animated into a sequence. In this case, though, the object is a circle and it's only meant to move very slightly, in order to show you how evenly distributed motion in a sequence makes the appearance of change of position in animation move much smoother in transition.