Reddish square: What does it mean

The red square doesn't mean anything; there is no analysis for highlighting squares. What probably happened was you accidentally right-clicked on the computer mouse and just so happened to right-click on a square (hence highlighting it red).
Right-clicking is just for chess streamers to highlight squares to aid in commentary/analysis. If your settings are turned on for it, you can even right-click drag to draw arrows on the board for the same reason.