An idiom is a figurative writing technique that usually uses the metaphor of completing a certain literal process. This hence prescribes a certain type of attitude to the individual in question. The idiom is 'willing to work from the ground up', Leroy in this case likely wont be physically building something from the ground up. But instead he will be metaphorically approaching the task at hand and is willing to put in all the hard work necessary to learn everything he can.
