What is it?
Horizontal Scale popup menu is used to zoom in and out on the sound being displayed.

Notes
When in Envelope mode the Horizontal Scale popup menu will be deactivated and the horizontal scale that best fits the envelope will be chosen.

You can also use the up and down arrow keys to zoom in and out.

Use the size box in the lower right corner to change the vertical scale.

To see all of the detail of the sound you must be zoomed in to at least ÷64.  Anything greater than that samples start being skipped to speed up drawing.  For example ÷128 only every other sample is drawn, ÷256 only every fourth sample is drawn.