Hello there, @Michael7979. Welcome to the Community space.
In QuickBooks, closing the books lets you set a password for a closed book.
Once you closed the books in your account, you aren't supposed to enter any entry for that fiscal year. But in QuickBooks Desktop, the system will allow you to enter transactions that affect the balance of the fiscal year, but it either tells you that it isn't recommended or ask for a closing date password if you set up one. That's the reason why it asks you to enter the previous year closing password.
To know more about the closing of books on your account, here's an article that you can read: Close your books in QuickBooks Desktop.
Keep me posted if you have other concerns. I'll be around to help.