Parabolic Stop and Reverse or Parabolic SAR was originated by the famous technician J Welles Wilder. The idea was to find changes in share price trends. Many investors use Parabolic SARs to set trailing stop losses on stocks.
Stockopedia uses the following defaults in the chart package for Parabolic SAR: Initial Acceleration 0.02, Incremental Acceleration 0.02, Maximum Acceleration 0.2
You can learn much more about Parabolic SAR at Wikipedia.