The average price or level over a specified period
Moving Averages is one of the oldest and most popular technical analysis tool to gauge price trends. Chartists, technical analysts, and economists have developed more sophisticated versions of the MA as well as apply it to not only prices but to numerous types of indicators as a method to smooth out the data.
The original and most simple version is essentially calculated by adding the closing prices of a specified period ie. 20 days and divide the total by the length of the period, here 20 days. After each new close, the olderst data is dropped and the new one is inserted. Before the computer era chartists would manually plot these averages out.