I will keep this very short but very educative. Talking about the idle mind i mean the effect of been idle. Been idle in this context doesn't mean to be lazy, staying at home doing nothing.
So many of us work at day but do also have free time. Then the idle mind comes in. What do you do whenever you are idle. Do you take out that time to think, gossip or spend that time in the internet doing nothing reasonable with your day? Been idle is inevitable but what you do with your idle time depends o you.
Idle mind comes with crazy thoughts. Either to think of how your college wore that skimpy gown just to seduce your boss or how that thought comes to plan an ambush on that crazy neighbor of yours. It could even be how that man promised to pay you if you compromise your beliefs.
But do you know that that energy you spend in nurturing those idle thoughts could also be used to achieve something great. That time could be used to profound that crazy multi million idea.