The draw tool can be accessed in the toolbar on the bottom right of the screen by all players, but only in Build mode. It allows you to draw shapes on the board. For each shape you have the option to choose the fill colour and border colour (if applicable).
The DM also has some special extra options in regards to the drawing mode, for more info see the documentation on Magical lighting.
The shapes that can be drawn are the rectangle, circle, polygon and paintbrush. We'll go briefly over each of them to explain some of the finer details.
One of the most simple shapes, a rectangle. While the mouse is pressed down a preview is shown of what the rectangle will look like if you release it.
Just like the rectangle, a circle is very simple. The drawn circle will be centered on the location you pressed down your mouse and will have a radius based on where you released the mouse.
Polygon / Line
This is probably the most complex shape, but it allows the most flexibility.
Unlike all other shapes you don't have to keep the mouse button pressed while drawing. Instead everytime you click somewhere on the board a new point will appear with a line connecting it to the previous point. You're free to cross or re-use pre-existing points.
To signal that the shape is complete you currently use the right mouse button. To cancel the drawing you can press Escape.
If you check the closed polygon checkbox, the last point and first point will automatically be connected upon finishing the shape and the polygon will be filled with the fill colour.
The last available draw mode is paintbrush, which is a freeflow option that offers you the ability to draw whatever.
Note that this is pretty bad performance wise, as it needs to use a lot of points to represent a shape. This is generally only advised for small things.