This means paying for something on a regular basis. Like, paying for Netflix every month. You would sign up for a subscription with Netflix for example, or Amazon Prime and pay a monthly subscription fee.
Another example would be paying for your cell phone service with a pay as you go plan. With this kind of plan, a customer would not sign a contract for a year, but they agree to pay a certain amount every month for their cell phone service, until they decide to cancel it. It's usually called a prepaid plan. For instance, I have a prepaid plan with my cell phone company. I sign up and agree to let them charge my bank account a certain amount every month. I can cancel at any time. But, until I cancel, the payments come automatically out of my bank account.
They are saying that if you can set up a business that includes customers paying a regular fee every month, such as a monthly subscription fee for Netflix or a prepaid cell phone service, or a monthly subscription to Amazon Prime, that this type of customer will bring in more profits than a business where you just sell them something new once in a while when they need it.
For instance, if you sell jeans, you would not be selling them a pair of jeans every month, probably. So, if you can think of some kind of business where your customers would need to pay you every month for your product or service, you will make more money.