Basically, its a little black row at the bottom of the screen (height adjustable) that would inform you of what your character is thinking right that second by way of text scrolling across it.
The ticker's font, size, color, etc. would differ depending on the situation. (by default the text would be white) For example, if someone is behind you and about to attack you and your character notices, the ticker could have a large bright red "BEHIND YOU!" appear so you as the player notice.
Additionally, it could also be used as a hint system. The ticker could scroll hints about whatever puzzle you're trying to solve, becoming increasingly more useful the higher your character's intelligence score is. (see: The Universal Hint System)