What is Bitcoin's locktime feature exactly? How can it be used? What is Bitcoin's locktime feature and what are some example use cases? 

And how can a user make use of this new feature? Is it in any gui wallet? And how would one use it in the command line if its the only option?
Like a cheque with a date in future, a Bitcoin transaction with locktime cannot be unlocked (read cashed) before that date. The definition in bitcoin.org is pretty clear


  Part of a transaction which indicates the earliest time or earliest
  block when that transaction may be added to the block chain.


Sample use case is a cautionary deposit. For a great example see micropayment channel protocol implemented by bitcoinj and described here: 

https://bitcoinj.github.io/working-with-micropayments