As the Dashboard column containers mandates the widgets stack from the top down, the spacer widget provides you with the ability to create some "space" between monitors if you wish to create groupings of them.
It is expected that you have already acquainted yourself with the two introductory Monitor pages, Monitors At Large and the Monitor Chooser. If you have not, please do so now to learn how we reached this point.
After double-clicking the monitor to put it into edit-mode, click the 'gear' icon [highlighted in green] to open the configuration window [highlighted in orange].
Alert Monitor Configuration's field descriptions:
The minimum permissible value for the Spacer Height is 35; the maximum value is 999.