Network fees must be added on top of your Bitcoin transaction in order to ensure that your transaction is confirmed and processed by the miners.
Bitcoin network fees depend on the overall traffic of the Bitcoin network. The current “advised” network fees can be found here.
By clicking on this link, you can see the number of transactions waiting to be confirmed on the Bitcoin blockchain and the network fees associated with those transactions. Remember that the network fee is displayed in Satoshi per byte.
The Bitwala wallet has a feature to manually set network fees (the fee slider).
There is a set minimum amount of network fees required to ensure that your transaction gets confirmed within a reasonable timeframe.