Like a technical indicator, a market indicator is a series of data points derived from a formula. In this case, however, the formula for market indicators is applied to the price data for multiple securities within the market, instead of just one security. Price data can come from open, high, low or close points for the securities, their volume, or both. This data is entered into the indicator formula and the data point is produced.
Unlike technical indicators, market indicators are not charted above or below the chart. Market indicators are what is being charted, and as such have their own ticker symbols. There are often many symbols that apply the market indicator formula simply to different markets. For example, the $BPSPX and $BPNDX track the Bullish Percent Index for the S&P 500 and the NASDAQ 100 respectively.