It is a period of times when penalties will apply if you cancel your cruise. They usually start once final payment is made.
One further clarification: Penalties start on the date final payment is due, whether or not final payment has already been paid.
If final payment is due May 1 and one made final payment on April 3, the money is still refundable on April 30. Similarly, if final is due May 1 and the passenger delays and doesn't actually make final payment until May 2 (assuming the cruise line doesn't cancel the booking), penalties still apply as of May 1 on the deposit.