The comment tool allows the adding of comments anywhere inside a level.
Comments can be used as a communication device between designers if multiple people work on the same level, reminders to yourself, and to explain to people outside of the map creation process your intentions.
To show comments in game, go to the console and type cl_comment 1.
This is a comment in the editor:
And this is what a comment looks like in the game:
Lets you specify the text that needs to be displayed.
Size of the text.
Lets you specify the distance at which the comment will be displayed. If the camera is further than this, the comment will be hidden.