This is the difference in the rate of change between two exponential moving averages.
The colors are green and red: - Green is when the fast EMA > slow EMA - Red is when the fast EMA < slow EMA
When the indicator approaches it's upper or lower limits, it will indicate either sideways price movement or a reversal. When the color changes around it's upper or lower limits from green to red or vice-versa, an EMA crossover just occurred and a continuation of trend is possible before a pause or reversal in price.