It's not a lookup from that index. It's a lookup to that index. Typically it will be 0 for nonclustered indexes (because key lookups aren't done to nonclustered indexes) and non-zero for the cluster (because key lookup are done to the clustered index)
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability