Each channel is different and has unique information relevant to the channel name. Some channels are content only, where you cannot message and only react to the content with an emoji to your liking. Other channels users can only react to the emojis presented and some channels have dedicated areas to message. If you get stuck in the Discord you can reach out to the community in the chatrooms or open a support ticket, by scrolling to the very bottom of the channel list and clicking ‘support-ticket’. Follow the support prompts in the channel.
For more information about Discord watch this video.