These scans are all based on the Stochastic Oscillator. It's a momentum indicator which is used to determine where the most recent closing price is in relation to the price range for a preceding period of time. This site uses the standard 14 day period (14, 3, 3) for its Stochastic calculations.

Stochastic tends to oscillate in a range between 0 and 100. Levels under 20 are considered oversold while levels above 80 are considered overbought. It's important to note that overbought or oversold can stay that way for quite a while given the right conditions. So it's often wise to combine Stochastic with some other indicators / analysis when making trading decisions.