Provisioning is the process which makes a Bluetooth mesh device a secure member of a specific mesh network. It involves a smart phone or tablet, running an application known as the Provisioner and results in the Bluetooth device being issued with a series of security keys. Conceptually it's comparable with Bluetooth pairing but technically is not the same and is concerned with the security of a whole network, not just the security of a pair of devices.
For a more technical explanation of Bluetooth mesh, download the Bluetooth Mesh Technology Overview document from https://www.bluetooth.com/what-is-bluetooth-technology/how-it-works/le-mesh
There are also numerous blog posts on various aspects of the topic:
Here's a quick demo of me provisioning a Silicon Labs Blue Gecko developer board which is running a Bluetooth mesh stack with software models that make it equivalent to a mesh light.