Testnet is the bitcoin test network that developers use to test their ideas, so they can make mistakes and not lose real bitcoin. If you aren't a developer, you don't want this option for the raspnode. The testnet blockchain is only 2GB so can be safely stored on the microSD card itself, unlike the real bitcoin blockchain.
Mainnet is the "real" bitcoin network. If you are getting a raspnode to either support the bitcoin network or to use it as a wallet for bitcoins, this is the option you want. Unless otherwise noted, all raspnodes will be on the "real" mainnet network.
The bitcoin core software is actually the same for testnet and mainnet. The default is for it to run on mainnet, but you can pass an argument that causes it to run on the testnet, so any raspnode could be used for either mainnet or testnet.