The Comment tool lets you place arbitrary text on the screen. When you first place the Comment tool, it contains the string "Comment...". To change the text, press the left mouse button on the tool. You will be asked to enter the new text.

The Comment tool automatically resizes itself to fit the text it contains. Even if you explicitly resize it to a larger size, the resulting size of the Comment tools will still be just enough to fit the text.

The Comment tool is very crude. It is limited to a single line of text.