Fixed Mode refers to the placement of the toolbars on the canvas. If Fixed Mode is selected then the toolbars will be on the left-side of the canvas and will be stationery. The default for the toolbars is Fixed Mode.
The toolbars will have to be turned off then back on again before the change to Fixed Mode will take effect.
The only way to turn off the Fixed Mode is to select Floating Mode.
How to use Fixed Mode