These are values that are computed using a variety of price-volume relationship formulae. The popular ones are MACD-Histogram, Relative Strength Index (RSI), Momentum, Stochastics etc.
For example, Momentum is an indicator whose value is computed by comparing the closing price of a share with the closing price of the share n-periods before that. The value computed is interpretted based on established rules.
Detailed explanations on these indicators are available in leading books on the subject of Technical Analysis.