Each column represents a warp thread, and each row a weft thread. The top and rightmost areas control the color, select a color in the sidebar and click on a square to change it.
The rows labelled with letters (A, B) represent pick up sticks. A filled in square represents a thread that is picked up. The row labelled as H1 is your heddle. The circles are the holes and the squares are the slots. Click on these to change between holes and slots.
The area on the right controls the weaving sequence. A heddle can be Up (▲), Down (▼), or Neutral (–); Neutral heddles replace down heddles when there are other heddles that are up or a pick up stick is in place. Click on these to cycle through the different positions.
The area in the middle is known as the drawdown and shows an approximation of the fabric.
For more detailed instructions, check out the docs or my YouTube Channel