Markers are highlights of squares on the chessboard. They can be used to show legal moves for your chess piece, the opponent's previous move, or a king in check. Their color and shape depends on which theme you are using. They can be toggled on/off in the general settings section of the configuration menu.
Clicking on your piece during your turn displays a marker on every square on which that piece can be moved. The picture shows the legal moves for the white queen.
When your opponent has made a move, their last move is indicated by markers. One marker shows the initial position of the chess piece, and a second highlights the new position of the chess piece.
If the opponent's last move put your king under attack (“check”), your king and all attacking pieces are highlighted as well.
Some themes may not support all types of markers.