What exactly do I need in a CSV file in order to import into the Passwords app?

Whatever info you want to associate with (and to identify) that password.

If the passwords are passwords for websites, I'd usually use the URL in that field. I've been storing freeform text in Keychain Access app for many years (with passwords for services or such that Keychain is unaware of, and can't pre-populate), and the equivalent of the label field here is the title of the secure note in Keychain Access app.

From the developer of the macOS 15 Sequoia Passwords app: "When adding passwords, you can specify a website or a custom label, like “Router”, "Passport Number”, or “Garage Door”."

PS: The shared groups feature is useful if you want to have (for instance) a few specific passwords you might want to share with other people.

Try exporting your passwords from Passwords app to get an example of the CSV file format expected.

You could repopulate that export file with your import data, too.

For me I was trying to import from Roboform into Password and I keep getting an error message about column headers and someone in the Beta group suggested exporting passwords from apple keychain to see what the headers were. I did that and changed the headers on the Roboform CSV file and was able to import with not further problem! The column headers on Apple passwords were

Title/ URL/Username/Password/Notes/OTPAuth

the column headers on Roboform were different or abbreviated. Hope this helps someone else!