A regular divergence is used as a possible sign for a trend reversal.
If price is making lower lows (LL), but the oscillator is making higher lows (HL), this is considered to be regular bullish divergence.
This normally occurs at the end of a down trend. After establishing a second bottom, if the oscillator fails to make a new low, it is likely that the price will rise, as price and momentum are normally expected to move in line with each other.
Below is an image that portrays regular bullish divergence.