The idioms 'to have your eye on the ball' or 'have your eye on the prize' mean to give all your focus and attention to something by being alert and ready. If you 'take your eye off the ball or take your eye off the prize it means to take your attention off something or to be distracted.

1. Even though Sarah did not enjoy her subjects and was required to work extremely hard, she got through college by keeping her eyes on the prize, which was the degree she was so desperate to get.
2. When I am cycling in a race I try to keep focused and not take my eye off the prize for one second.

1. Leading this project means a lot of pressure and responsibility, so I really need you to keep your eye on the ball on this one. No procrastinating! 
2. It took Frank years, but eventually he got his business up and running. He put his success down to the fact that, through his many setbacks, he always kept his eye on the prize.
3. Okay, everyone, I need you all working very hard to reach your targets. Whatever you do, don't take your eyes off the ball this month, please.