Activation is making a phone active on a cell phone network. It is the process that links a specific cell phone to a specific customer account and phone number.
It requires configuration on the network side, for the network to recognize that specific phone as belonging to that customer.
It may also require configuration on the phone itself. This may be automated to some extent, or it may require following special instructions to enter specific codes into menus that are normally hidden.
Phones that accept a SIM card can usually call and text immediately after inserting an active SIM card. But if the SIM card is new, the SIM card may need to be activated first. And more advanced features – beyond basic calling and texting — may still require that the phone is registered with the network through a specific process like activation.