At Copyright Agency, 'rights reversion' means a publisher is no longer entitled to receive Copyright Agency payments for a book title, and future allocations will be paid solely to the creator/s.
Some publishing contracts state that rights ‘revert’ to a writer or illustrator in specified circumstances (for example, the right to republish when the book is out of print) but expressly state that the publisher retains entitlement to a share of any future Copyright Agency payments.
Check your publishing contract to understand the circumstances, if any, in which the publisher’s entitlement to a share of Copyright Agency payments ceases.
See also: How do I tell you that the rights have reverted to me?