Yes, your key does not leave your browser.. as long as your computer is secure your key is too.
The XIM wallet only shows your accounts XIM balance and no other assets from your account including Lumens (XLM).
The USD value on the right side is the current live price value of all your accounts XIM.
Transactions are powered by tiny fractions of lumens (XLM), for example one Lumen is worth .40 cents and can power 100,000 transactions so each XIM account has one or two lumens to power all its future transactions.
You do not have to pay much attention to this value as it will likely never be reached.
XIM is easily converted to local currencies through many exchanges, for example you can find your currency at
StellarTerm.
In the near future XIM will be including its own local currency anchors directly into our wallets for your convenience.
