Carrier settings on the iPhone are stored in a file containing the configuration for cellular data and calling. Carriers, such as AT&T or Verizon, may include settings for calling, messaging, personal hotspot and other items in this file.
The iPhone will occasionally ask to update carrier settings, which will optimize the cellular connection on the iPhone.
Apple defines carrier settings as follows:
Carrier settings updates are small files that can include updates from Apple and your carrier to carrier-related settings, such as network, calling, cellular data, messaging, personal hotspot, and voicemail settings. You may receive notifications from time to time to install new carrier-settings updates.