A 12 period rate of change compares today's close to the close twelve bars back. Positive means the market is higher than it was; the size tells you by how much. No smoothing, no normalisation.
Its simplicity is a virtue: there is nothing hidden in the calculation, and it forms the basis of cross-sectional momentum strategies in portfolio research, where the twelve month version is among the most studied anomalies in finance.
On a single chart it is noisy and its swings depend entirely on the lookback-period. It also produces artificial signals when an old extreme value rolls out of the window, an effect worth remembering before reading meaning into a sudden change.
Related: momentum-indicator, lookback-period, true-strength-index, percentage-price-oscillator, divergence