There are a number of different errors that the Quality check will show. One of the errors you can encounter is of the format 'id'/'gtin'/'ean' contains values that are not unique.
Identifiers such as id, gtin, and ean codes should be unique for every item, meaning that there should not be multiple items that have the same identifier. If it does happen that one of these identifiers is shared between items you will see this error.
There are two ways to solve the error. If the items that share the identifier are the same item, you can use a deduplicate items rule. However, if you want to keep both items because they are two different variants of the same product you need to change the identifiers in your import file.