A keyword is a TEXT or a Phrase that your contacts can text to the shortcode number. They will be added to your list which allows you to interact with them. Sending a Keyword will be an OPT-IN or subscribing for the campaign.
Example: While running for mayor the candidate invites subscribers/voters to join his campaign to get all updates by sending VOTE to 33339. Voter sends VOTE to 33339 and gets added to a list to get updates.