What does this diode symbol mean?
The title of the datasheet you linked is, "TLVH431, TLVH432 Low-Voltage Adjustable Precision Shunt Regulators". It's a kind of programmable Zener diode.

The description tells us 


  The TLVH431 and TLVH432 devices are low-voltage
  3-terminal adjustable voltage references, with
  specified thermal stability over applicable industrial
  and commercial temperature ranges. Output voltage
  can be set to any value between VREF (1.24 V) and
  18 V with two external resistors (see Figure 19).
  These devices operate from a lower voltage (1.24 V)
  than the widely used TL431 and TL1431 shuntregulator
  references.
  
  When used with an optocoupler, the TLVH431 and
  TLVH432 devices are ideal voltage references in
  isolated feedback circuits for 3-V to 3.3-V switching-mode
  power supplies. They have a typical output
  impedance of 0.25 Ω. Active output circuitry provides
  a very sharp turn-on characteristic, making the
  TLVH431 and TLVH432 devices excellent
  replacements for low-voltage Zener diodes in many
  applications, including on-board regulation and
  adjustable power supplies. [Emphasis mine.]




From the datasheet.

The flyback power-supply is one application where it is useful. We need to signal from the secondary side back to the swithing controller on the primary side when the output voltage reaches the required voltage. Opto isolation is good and the TLVH makes it easy. The control pin is referenced from a voltage divider on the output and the resistor values are calculated to give 1.24 V on the control pin when the output voltage is correct. The device then turns on allowing current to flow through the opto-LED.