Sticky messages are messages that stay at the bottom of a channel. You can create messages that users will see before sending a message. Check out this gif for a good example:
Inside the channel you would like to create the sticky message in, run
ch!sticky message with message being the message you would like to stay at the bottom of the channel.
And boom! You're all done!
To turn off the sticky message in a channel, run
ch!sticky off in the channel.