But WhatsApp is free. What does WhatsApp conversation charges mean?

Yes, WhatsApp as a messaging app or a business app is free.

However, for businesses looking to start communicating to their customers and grow their sales at scale, WhatsApp Business API platform is the best fit. To leverage WhatApp Business platform as a sales channel, there are conversation charges levied by WhatsApp.

When you send a notification to your customer via WhatsApp Business platform, they are chargeable if your customer has not replied in the last 24 hours. The charges depend on the country code of your customers’ WhatsApp accounts. We bill these charges on actuals to you without any surcharge.