In this selection you can change the appearance and size of the four main windows, and select the active one. Most probably you will always keep the currently active window maximised and use the Tile or Cascade selections only when a general view is required.
You cannot close any of the four windows (Components, Courses, Weeks, Timetables), but you can minimise some of those (by clicking the minimise button on top right corner). Those windows that are not minimised, are affected by the Cascade and Tile commands.

Arranges open windows in an overlapping pattern so that the title bar of each window is visible.
Arranges windows side by side so that all windows are visible. The active window is positioned in the top left corner.
You might like to minimise Weeks and Timetables, leaving Components and Courses views open. When you now click Tile command two or more times, the two windows appear in turn side by side an on top of each other. It is sometimes useful to keep these both windows visible at the same time, click a code on the window and then the frame on the other. Mimosa remembers the last code and activates it automatically in the other window, enabling to cross-check.

Arranges minimised windows into row.
Minimises all windows into icons.
Restores all minimised windows to their original size.
RefreshPerforms all pending calculations and refreshes all windows.
Activates next view (Components -> Courses -> Weeks -> Timetables -> Components).
Activates previous view (Components <- Courses <- Weeks <- Timetables <- Components).
Activates Timetables view.
Activates Weeks view.
Activates Courses view.
Activates Components view.