This group of DMVs records every scan and large key lookups. When the optimizer declares that there isn’t an index to support a query request it generally performs a scan. When this happens a row is created in the missing index DMV showing the table and columns that were scanned. If that exact same index is requested a second time, by the same query or another similar query, then the counters are increased by 1. That value will continue to grow if the workload continues to call for the index that doesn’t exist. It also records the cost of the query with the table scan and a suspected percentage improvement if only that missing index had existed. The below query calculated those values together to determine a value number.
Click through for sample scripts for this and the index usage stats DMV. The tricky part is to synthesize the results of these DMVs into the minimum number of viable indexes. Unlike the optimizer—which is only concerned with making the particular query that ran faster—you have knowledge of all of the queries in play and can find commonalities.