When you send funds from your bitcoin wallet, the specified amount of funds are sent to the intended bitcoin address and the remainder of the funds being stored in the sending bitcoin address are sent to what is referred to as a "change" bitcoin address associated with the same bitcoin wallet. The change is not available to spend until the bitcoin transaction gets confirmed on the bitcoin network (funds get delivered to the intended address).
Consider the example of taking a $20 bill out of your wallet to pay for a $5 cup of coffee. You would give the $20 to the cashier, and the cashier would give you back the $15. While the $15 belongs to you, it is not available for the time it takes the cashier to give you back your change.
You can choose to spend your change before your original bitcoin transaction confirms, although it is not recommended. To change this setting in your wallet, go to Settings, then Advanced to turn on Use Unconfirmed Funds. Note that it will take longer for bitcoin transactions with unconfirmed inputs to get confirmed on the bitcoin network regardless of the fee included with the transaction.
Please see the following Bitcoin Wiki article regarding how change addresses work: