This acronym means Moving Average Convergence Divergence and the indicator is calculated as the difference between a shorter and a longer moving averages.
The realized volatility is a measure of the average size of the returns offered by a security in a determined period. For example, a stock that falls or rises 30% in a few days is considered very volatile, and conversely a Treasury bond that moves 1% at most on a given week has low volatility.
Volatility by its nature is a range-bound measure on the low side, it can never go below 0. This allows traders to distinguish regimes of low volatility.
The more appropriate name for this measure would be “standard deviation of returns” but financial jargon
Volatility is used in two different settings at least: