the Grand Staff
Now that you've practiced a bit with you left and right hands independently, the Grand Staff puts them together. From the Complete Summary, you saw that it is comprised of the treble and bass staves. Both are synchronized timelines along which the notes flow, and as a result it is easy to see the time relationships between the two.
Time relationships between the staves is not a challenge in the first few examples of scores in this lesson. As shown below in the Mexican Hat Dance, while both staves have notes, but there is no overlap in playing between the two staves. The idea is just to get you used to the idea of seeing both staves and knowing which hand to use to play the current notes in time. You can see the first 3 measures in both groups of 4 are the same, only on different staves. Additionally as in the earlier lesson, after placing your hands in the C position, you can play the notes on both staves without having to move your hands left and right.