Fixed Width enables you to fix your frame to a maximum width of 1280px.
Fixed Width is most commonly used if you would like to contain your text on desktop devices (e.g. screens over 1280px wide). This offers polite consideration to your desktop users, so that the reading width of your page, does not span the full width of the screen. For large desktop devices (e.g. 1920px wide), applying no fixed would make reading unpractical. Therefore, 'Fixed Width' need only be turned 'On' in one frame per layout. When you set Fixed Width 'On' in one frame, all other frames within the Row will be set to 'Off' automatically.