Nav Coin is a Proof of Stake cryptocurrency. This means you mine Nav Coins by staking. This does not require any fancy equipment or expensive hardware but can be done on a device as light as a raspberry pi. The NavPi is just the name for a Raspberry Pi 3 device running the Nav Coin software for mining. You can buy the NavPi, a raspberry Pi with pre-installed software from the Nav Coin store (http://store.navcoin.org/), or build one yourself