An index is considered Inefficient if it has many updates, and not so many seeks, scans or lookups. This is an indication that the index isn’t being used much and that it is being updated often.
Just because an index is inefficient, doesn’t mean it should be deleted, it should be considered for removal. It might be that an index is inefficient but it is used in a report that the CEO uses once a month, in which case it should probably not be removed.
You can find the opposite of this report, the most used indexes in the Most Used Indexes report.