This toggles the underlining of (possible) spelling mistakes, new to v5.1.0. Previously, toggling this required opening a text windows to be able to access the Edit->Spelling submenu to toggle the 'check as you type' feature.
On low-powered systems and/or with documents with large amounts of $Text leaving this off may help with performance. Otherwise, leaving the setting off simply avoids the red underline spelling prompts. The latter aren't helpful if showing the app in a presentation or if using multiple languages in $Text.
Default: not ticked