Every device needs a license
Meraki hardware is managed from the cloud dashboard, and the dashboard only manages devices covered by a valid license. The license is a separate purchase from the hardware, with its own term, commonly one, three, five, seven or ten years.
Licenses belong to an organization
A license is applied to a Meraki organization, not to a physical box. That's why a used device never brings a usable license with it, and why licenses are bought from a Cisco partner for your own organization.
Know your licensing model
Meraki organizations use one of a few licensing models, such as co-termination, where every license in the organization shares one expiry date, or per-device licensing, where each license has its own term. Adding licenses to a co-termination organization changes that shared expiry date.
Your dashboard shows which model you're on under Organization > License info. Check it before you order, because it decides which license SKUs you need.
What happens if licenses lapse
An organization that falls out of license compliance gets warnings and a grace period. If it isn't fixed, devices stop working. Don't add hardware without covering it.
Questions to answer before you order
Which licensing model is the organization on? Which license edition do you need (for example Enterprise or Advanced Security on MX appliances)? What term? And when do your current licenses expire? With those answers, the hardware and license can be planned together.