By default, the trigger area for the button is your content. That means, if you have small narrow text, your user's mouse will have to be right on the individual letters to trigger the button. The Hit State will fix this problem by allowing you to designate a bigger trigger area.
Add a keyframe in the hit state then draw a shape that matches your desired trigger area. This shape will not be visible in the final product. Fill and stroke color do not matter, however any solid area will react as the hit area.
Open the demo saved as Buttons/demos/TextButtons.fla. Drag the playhead to the hit frame and you will find the shape setting the trigger area.