RCS stands for Rich Communication Services, it's a newer messaging protocol that's meant to replace traditional SMS. I've been using it for a while now and it's definitely an upgrade, you get features like end-to-end encryption, group chats, and the ability to send larger files and higher quality images. It also supports read receipts and typing indicators, which is pretty handy. The main difference between RCS and SMS is that RCS uses your internet connection to send messages, whereas SMS uses your cellular network, so you need a data plan to use RCS.