The online index feature provides a powerful way to perform maintenance operations such as rebuilding or creating indexes in a production system without sacrificing DML concurrency.
This feature allows concurrent modifications (updates, deletes, and inserts) to the underlying table or clustered index data and any associated indexes during index data definition language (DDL) execution. For example, while a clustered index is being rebuilt, you can continue to make updates to the underlying data and perform queries against the data.
Users are not blocked from querying and updating the underlying table during the index operation. Check these link for more details: