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".