The numeric buttons across the top of the grid trigger highlighting. When a numeric button is clicked, the background turns red for that button & all the cells with that digit turn green. Button 1 was clicked below (red background) & all the cells with a 1 are highlighted in green.
This assists in identifying sub-blocks with a single row or column containing the target digit. In this case Row F has a single row with 1's which means only that sub-block can have a 1 in that row, see rectangle D-E/7-9. Thus the 1's in F4 & F6 can be removed.
By sequentially clicking each numeric button, each digit can be inspected & any Sub-Row/Column Eliminations can be made.