Tarrasque.io provides you with a quick draw tool to help you sketch out parts of the map or provide clues to your players.
First, select the Draw tool from the Map Tools. The freehand draw menu will appear above the bottom bar.
You can now start drawing on the map by clicking-and-dragging your mouse cursor/finger. If you'd like to use a different color than the default, simply open the colorpicker and choose another one. You can also switch to the eraser tool to delete any lines you've drawn that you no longer need.
If you want to remove all drawings on a battle map, you can click the Clear All icon on the right (GM-only).