What is ethereum harmony for?
AFAIK, Ethereum Harmony provides a interface (includes a wallet and a command line interface) to interact with ethreum networks by running a node and it uses ethereumJ which is a Java implementation of ethereum protocol. It can be used to connect to main ethereum network, test networks or a private network.
Could you suggest a good starting point to get into Ethereum Dapp
In Dapp Development you need to learn two parts; smart contract development and the Dapp Interface development. As I feel it's always better to start with contract development.
Here are the dapp developer resources mentioned in the ethereum github wiki. Above process will help you to be familiar with these resources.
NOTE : I assume you have a sufficient idea about what is blockchain and ethereum. If not as the very first task you may learn the basics of blockchain concept and the ethereum implementation (contracts, accounts , mining etc.). If not you may start reading the ethereum white paper and this ethereum development tutorial before anything.