Mobile Keywords are unique words that you create for your customers to text to join your SMS marketing program. They offer a quick and effective way to build your contact list, and allow your customers to opt-in to receive coupons, special event invites, or any other form of promotion for your business. Since your customers are willingly consenting to receive messages from you, they'll be less likely to opt-out. For example, a coffee shop could create the keyword BEANS. Their customers would opt in by texting BEANS to the short code 77453.
You need to make sure your keyword is widely visible. It will never be effective if no one sees it.