Reserves refer to the ERC-20 contracts of the underlying assets.
Since ETH is used directly in the protocol (instead of an abstraction such as WETH), we use a mock address to indicate ETH: 0xEeeeeEeeeEeEeeEeEeEeeEEEeeeeEeeeeeeeEEeE
For assets on testnets, we sometimes use another version of the token (e.g. testnet Dai). This is to ensure enough liquidity for our reserves and to easily mint more tokens when needed.
If you are developing on a testnet and require tokens, go to faucet: http://testnet-v1.aave.com/faucet, making sure that your wallet is set to the relevant testnet.