Index coverage is a report in webmaster tools that shows how many pages are indexed, how many have been crawled without being included in the index, and for what reasons the rest were excluded. It is the fastest way to spot technical problems: a large number of pages marked as crawled but not indexed tends to signal thin or duplicate content, server errors point to performance problems, and pages excluded by a canonical tag point to incorrectly configured canonicalisation. It is worth tracking the trend over time in particular, and comparing the number of indexed pages with how many the website should actually have. A significant difference in either direction calls for an explanation. It is worth checking index coverage at least once a month, so that a problem is caught before it shows up as a drop in traffic.
See also: Index bloat, Orphan page, Thin content.
