Are Bitcoin fees optional? I was making a multisig account in coinb.in. Then I tried to make a transaction (spend funds). At the bottom there was a box for writing the fee. 

What fee? I thought bitcoin sending was free! Beside the box, it said, "Make sure you include an adequate fee, otherwise your transaction may be rejected. 0.0005 BTC or more is recommended."
The fee is optional; however, including your transaction in the next found Bitcoin block is also optional.

The miner who finds a block gets to decide which, if any, transactions are recorded with the block they found.

Factors that influence whether your transaction is included in that found block include:


The block size limit.  Sometimes the transactions are too large to all fit into the block, so most will prioritize higher fee transactions to be included first.
Local settings for the person finding the block can be set to ignore transactions with fees lower than an amount of their choosing.
Some people will include all the transactions they have seen, as long as they will all fit, but this is not the normal default mode of operating for miners.  If you do not include any fee, then this would be your only chance.