The text or leader tool in SketchUp is used to create notes as needed in the model. By default there is no keyboard shortcut for this tool. The way to activate it is by using the toolbar button or selecting Text from the Tools menu at the top of the window.
Once the text or leader tool is activated you can click on empty space near the model to create an on screen text box. This will allow you to add notes or label certain things in the drawing as needed but not directly point to anything in particular.
If you wish to specifically point to an item in the model you can do that as well. Activate the text or leader tool and click on a point, line, or face of the model and select where you want to drop the text box. Then type in the appropriate information you wish to use and either press enter twice or click outside the box. This is handy for creating notes that are specific to a certain part of the model.