While Readable is speaking, the current word gets a coloured underline directly beneath it, so you can follow along by eye. The underline is on by default, in red, 3 pixels thick.

Sample text in TextEdit with a paragraph selected and a red underline under the word being spoken; the toolbar shows a red Stop button.
Readable reading a selected paragraph in TextEdit. The current word is underlined in red, and Play has become a red Stop button.

Changing the underline

Open Settings and go to the Reading tab. The underline settings are under Visual tracking:

Underline each word as it is spoken
Untick to turn the underline off.
Underline colour
Click the colour swatch to pick a colour.
Thickness
From 1 to 10 pixels.