You can control a computer either in a window, or using the entire administrator computer screen. The "Fit screen to full display" toggle button changes between these two modes.

In full screen mode, the client computer screen is scaled up to completely fill the administrator screen. In addition to the client screen, there are a number of Apple Remote Desktop controls still visible overlaying the client screen.

In in-a-window mode, you can switch between fitting the client screen in the window or showing it actual size, possibly scrolling around the window to see the entire client screen. See Switching the Control Window Between Full Size And Fit-To-Window for more information.