Advance/Decline Index

What Does Advance/Decline Index Mean?
A technical analysis tool that represents the total difference between the number of advancing and declining security prices. This index is considered one of the best indicators of market movements as a whole. Stock indexes such as the Dow Jones Industrial Average only tell us the strength of 30 stocks, whereas the advance/decline index can provide much more insight into the movements of the market.

Investopedia explains Advance/Decline Index
In general, rising values of the advance/decline can be used to confirm the likelihood that an upward trend will continue. If the market is up but there are more declining issues than advancing ones, it's usually a sign that the market is losing its breadth and may be getting ready to change direction.

Imagine that the advance/decline index on the S&P 500 is currently at 1835. If at the end of the last trading day, 300 stocks were up (advance) and 200 were down (decline), 100 would be added to the advance/decline index value, pushing it to 1935.

When this index is plotted on a chart, it is known as the "advance/decline line".