In the admin area you get a new text field.
When a customer logs in after you have changed and saved the text field, a pop-up appears with your text. Below this there is a button "agree" or "accept" or however you want.
If the customer has pressed the button, they will not receive a pop-up the next time they log in, until you enter a new text.
Possible applications:
- Notification of price changes
- new free shipping
- new shipping costs or payment methods
- holidays
- extended delivery times
- etc etc.
A more cost-effective option would be to redirect customers after login.