What does this denote in the schematic?
That's called a potentiometer, or "pot" for short.

It is a fixed resistor between the two ends shown top and bottom in your schematic.  The third terminal, shown with a arrow in a standard schematic symbol, is a wiper that contacts the resistive strip in a place that depends on how the pot is adjusted.

The net result is a fixed resistor end to end with a variable tap someplace within that resistance.

One use of these things is to make a variable voltage divider.  Feed the fixed voltage into one end, ground to the other, and a variable voltage comes out the tap depending on how the pot is set.

This is also the basis for most old volume knobs.  One wrinkle with adjusting volume is that humans perceive volume logarithmically.  There are special pots called "log taper" where the resistance along the fixed resistor varies logarithmically, not linearly.  If a pot isn't specified to have log taper, then you can assume it has linear taper.  Log taper pots are harder to find now that volume controls are generally implemented digitially.

In your schematic, the pot is used as a rheostat.  That's a 2-terminal variable resistor.  That's what you get when you short the wiper to one of the ends, or leave one of the ends open.