Channel elements are all the little individual components that make up the face of the chatbot as well as the logic. Use the drag-drop editor to build and configure your chatbot according to your use case.
There are many send elements but availability depends on what is supported channel side. Select a channel to filter results by channel. Also make sure to check out the Message types sections of channel docs for individual limitations as they differ from each other.
To enable a channel, view the instructions in the documentation for that channel.