Click on red parts of the text to see Granska reports.
When the "Display" checkbox above is checked, a new text area will show up over the part where the Overleaf rendered results are shown. If you uncheck the checkbox, this overlay disappears again.
The results of running the grammar checker Granska on the text content of the LaTeX code are shown. Words shown in red have some message from Granska, and you can click on them to bring up a window with information. If there are suggested corrections from Granska shown in green, clicking a suggestion will update the text with this suggestion and then try to merge that change back into the Overleaf LaTeX.
Sometimes merging changes back into the Overleaf LaTeX goes wrong. Things that can cause problems include suggested corrections with multiple words where the original LaTeX had some markup between the words. For example, if we have the original text "ute i en \textit{trädgården}", Granska will suggest changing "en trädgården" to "en trädgård" but when merging the change back into the LaTeX the result is "ute i en trädgård}", where the markup is lost and the brackets are no longer correct so the result is not even correct LaTeX.
If the text includes characters that are not present in the Latin-1 character set (perhaps in foreign words quoted in the Swedish text etc.), these characters will likely not be handled properly and they may cause the plugin to believe a correction has been made that simply removes these characters.