The two lines of the Vortex Indicator (uptrend VI+/downtrend VI-) are used to spot trend reversals or confirm current trends. The indicator was developed by Etienne Botes and Douglas Siepman. Included in the calculation True Range to take into account volatility. This makes the indicator a comprehensive volatility-normalized positive/negative trend movement indicator.
The bigger the difference between the low of a bar and the next bar's high, the stronger the upward vortex. The bigger the difference between the high of a bar and the next bar's low, the stronger the downward vortex. Signals are often seen at crossover points for the two lines.
Read more about the Vortex Indicator at Investopedia