This warning has been raised because this index is superseded by another one. This means that, for example, if this index is on columns A and B, then there exists another index on columns A, B, and C. Dropping this index will mean queries use the remaining index, even if they don't query on column C.
If this index has one or more included columns that the other doesn't, then Aireforge Studio will suggest merging the indexes, to create a new one with the indexed columns of the wider index but the included columns from both. If this index doesn't have any additional included columns, then it will suggest dropping this index.
Consider removing the superseded index.
Any suggestion from Aireforge should be thoroughly tested before being implemented into production.