What is an HD wallet, and why did you decide to make Jaxx HD?

Static (non-HD) wallets stick to using a single address for all transactions. While that's simpler, address reuse greatly undermines your privacy. 

HD (or "hierarchical deterministic") wallets like Jaxx generate a new address every time funds are sent to the current address. All addresses generated from the same wallet can be used to receive funds, and all funds sent to any of these addresses will show up in your Jaxx balance. The balance you see on your main wallet screens represent the total sum between those addresses.