RCS is a feature that's being rolled out across the board on all networks*in the near future. It's a more reliable way of texting and will replace the native messaging capability. You can find out more about it*here.*￼
The HTC One is the first handset to get this service via a software update. You'll be offered to download it from the maintenance release mentioned in other posts. Once installed, it can't be removed but it can be disabled in the phone settings.
Like NFC, Bluetooth or Wi-Fi, RCS feeds can use more power on your phone so those who use the service (or a similar option like Whatsapp) may see increased power usage.
From some forum
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