HP Instant Ink is a subscription service provided by HP for its range of printers. This service delivers ink cartridges directly to a user's door based on their printing usage and needs. Subscribers choose a monthly plan based on the number of pages they expect to print each month, and HP monitors printer ink levels. As the ink runs low, replacements are automatically sent, ensuring that users always have ink available. The service is designed to be cost-effective, offering recycling options for used cartridges and potentially reducing the cost per page compared to regular ink purchases.