What is the difference between ‘first to invent’ and ‘first to file’?

With the passage of the Leahy-Smith America Invents Act, in 2013 the US patent system transitioned from a “first to invent” to “first to file” system and changed the focus from when an idea was first conceived to the date when an invention was first submitted to the US Patent and Trademark Office as either a provisional or non-provisional patent application.