@ 11:10, in the video tutorial, the hot keys for pressing Y and can we e press P ?
This won't be the best explanation, but here goes. When you press P you create a new mesh object, when you press Y it is still part of the original object. The easiest way to see it is to try it yourself,
create a cube
go into edit, subdivide the cube
on one section use P and another use Y, you can no longer modify the P one, but you can the Y one
go into object mode, move and scale the P one and nothing happens to the Y
also look in the outliner tab area, the P created a new object and the Y doesn't.
Hope this helped.
Yup... "P" separates the mesh into it's own object and "Y" splits the mesh, but retains it's position within the same object.
And as Tanya suggests, sometimes playing around with simple primitives to see what each does differently is the best teacher.