A 200-day Moving Average (MA) is simply the average closing price of a stock over the last 200 days. Moving averages vary in their duration depending on the purpose they are used for by stock traders. Moving averages are trend indicators of price behaviour over some time.
The 200 Day Moving Average is a long term moving average that helps determine the overall health of a stock. A 200 Day moving average is calculated by taking the closing prices for the last 200 days of any security, summing them together and dividing by 200.