What is a Mining Fee?
When broadcasting a Bitcoin transaction, you must first add an appropriate mining (transaction) fee to it. This fee is paid to give an incentive to Bitcoin miners, who are confirming blocks into the blockchain. If you would like your transaction to confirm quickly, say in the next block mined, then you would need to choose a fee which puts you into a higher priority tier over other transactions. If you are in no rush to complete your transaction, you may choose an accordingly lower mining fee, your transaction will then confirm in a later block, as its lower fee offers less of an incentive to miners.
The mining fee is paid in Satoshis/byte or kilobyte. For example, you may set your fee to be 30 Satoshis/byte, which would also be described as .0003 Bitcoins/kilobyte. This is what miners look for when determining whether they will include your transaction in the current block they are attempting to mine (confirm). Each block can only contain 1 million bytes (1 MB), so those transactions with the highest fees applied, hold greater priority for miners to include in the current block they are mining. There is nothing says you need to include a mining fee at all, however if you choose this option you're likely to have a transaction that may not confirm for days, weeks, or even an indefinite amount of time. One of the best sources to use when determining what your mining fee should be can be found here: bitcoinfees.earn.com/#fees. This tool gives a current assessment of what fee should be applied to have your transaction confirmed in the amount of time that you desire. To give an example, if I send a transaction that is 223 bytes (a median-sized transaction) and determine that currently 9 Satoshis/byte will likely allow my transaction to confirm within my desired window of time, then the to total mining fee I will be paying will be: 9 x 223 = 2007 Satoshis, or 0.00002007 BTC total. If you're using a wallet that predetermines what your fee should be, based on the priority settings you have applied, double check the calculation to be certain that their current rate matches the information listed at the link I provided above.