Choose Reading ruler from the More ▾ dropdown to open Sensory Ruler for Mac — a coloured strip that follows your cursor to help you keep your place while you read.
Sensory Ruler is part of the Sensory Readable Suite, and can also be launched independently from its own app icon at any time. The Reading ruler item only appears in the More menu when Sensory Ruler for Mac is installed.
For how to set up the ruler, see the Sensory Ruler for Mac help.
Video help
A short video showing this feature in Readable for Mac v2. The video only contacts Vimeo once you press play.