In a layout, switches from paper space to model space in a layout viewport.
Commands operate in either model space or paper space. You use model space (the Model layout) to do drafting and design work and to create two-dimensional drawings or three-dimensional models. You use paper space (a named layout) to create a finished layout of a drawing for plotting.
When you are in a layout, you can enter mspace at the Command prompt to make the last viewport in the layout current, and then work in model space in that viewport within the layout.