Placeholders are a way of letting FlowJo work with graphs inside a layout without having to calculate the image while you are trying to work. 

The time to calculate a graph of a large data set can be several seconds.  In a layout, which may contain several to dozens of graphs, this recalculation can be very time consuming.  Therefore you are able to tell FlowJo to defer calculation until later, and to represent the graph as simply a box with diagonals running through it.

Use the checkbox in the top right area of the layout editor to tell FlowJo whether to use placeholders or not while you are working on the layout.  Note that this setting only applies to the layout editor itself.  Whenever you produce a stack of frames, the graphs are fully calculated regardless of this setting.

There are a couple other times when placeholders may be drawn in the layout editor, even when you don't have the Use Placeholders box checked.  If the layout is zoomed way out (less than 25% magnification) then text boxes will be turned into placeholders.  Also, if any of the graphs in the layout are based upon parameters which are not available within the current iteration value, or it is otherwise impossible to draw a graph based on the data available to FlowJo, then it will simply draw the placeholder instead.

Use the checkbox in the Tables/Layouts pane of the Preferences in order to set the default state of the Use Placeholders checkbox.