Summary:
In a Repeatable Read isolation level, new rows can be inserted into the dataset.
In a Serializable isolation level, all the rows are locked for the duration of the transaction, no insert, update or delete is allowed.
Please refer to these for examples and explanations.
Original links are broken — Wayback Machine links are from the same day and time.
Isolation Level — serializable
Wayback Machine SQL Server Transaction Level Serializable
Isolation Level — repeatable-read
Wayback Machine SQL Server Transaction Level Repeatable Read