ASCII art uses monospaced fonts to draw pictures and diagrams. It is commonly useds in online comments and readme files where you cannot insert image files.
The editors functions similarly to an image editor. Clicking anywhere in the textbox will position the cursor. Typing does not move the existing image, but will overwrite any existing characters.
The Paint tool allows you to draw freely. You can change the letter to be drawn in the sidebar. The Erase tool sets the region to blank. You can adjust the size and shape of the brush in the sidebar.
The Select tool allows you to select rectangular regions. Switching to the Move tool allows you to move the selected region around by clicking and dragging. Typing whilst you have a selected region will fill the entire area with the character.
The Rectangle and Circle tools allow you to draw shapes easily. Holding the shift key will make a square or a circle.
If you want to align different parts of your image you can toggle Gridlines, which will add a grid outlining the place on each character.
To copy only a part of your diagram, first highlight it with the Select tool, then press Ctrl + C.