Take a quick look at what different move marker types mean. There are five move types that you can see in the game.

Normal moves (yellow default color) mark cells where you can move safely, that is, being in that position, piece will not be under attack of any opponent's piece.
Optimal moves (star with green default color) marks cell with optimal move according to last used nouse algorithm. It helps to not to do trivial mistakes if you are new to chess game. Nouse algorithm takes into account new position attack potential, covering own pieces, whether move position is covered and more other things. Read about nouse algorithm in more details on dedicated page.
Understrike moves (red default color) mark cells that understrike of some of opponent pieces. This helps to plan your next move.
Understrike opponent pieces (red default color) - those opponent's pieces that you can beat with selected friendly piece. Make sure that beating makes sense and your piece will not be lost.
Covered friendly pieces (green default color) - those friendly pieces that you cover with selected friendly piece.

There is also a possibility to show wire structure on all cells that your move markers placed in. This helps a bit with understanding perspective. This feature can be turned on in game preferences window.

Currently selected piece is showed with yellow default color and bigger scale. You can select opponents pieces too and see their covered/understrike pieces. This helps to understand current situation and make right choice.