what does it mean to unbrick my iphone?

Apple refers to activating a device as "unbricking." Your "last unbrick date" is the last date the device was activated with a carrier according to Apple's systems. As this information is always sent to Apple from all carriers that want to activate devices with a carrier profile (device settings specific to each carrier). If you activate a device with multiple carriers through its life (take it from T-Mobile to Aio Wireless for example), it will have multiple unbrick dates in the device history maintained with Apple.

A cell phone that is "Bricked" is considered to be unrepairably damaged electronically, usually not from physical damage. To "unbrick my iPhone" would mean to get a non-working iPhone working again.

It means the iPhone has been jailbrokern or otherwise modified in such a way that it no longer works and cannot be restored without completely resetting it. This assumes that no damage has been done to the device's ROM.

Or it may simply require reflashing a ROM. Depends on what exactly "bricked" means in this specific case because the word is often misused by people who don't understand the usual context.