"Keep your eye on the ball" is an idiom that means to stay focused and attentive on the most important goal or task at hand, remaining alert to the situation and not getting distracted by less important things.
The phrase originates from sports, like baseball, where a player must watch the ball to be successful, and is now used in a general sense for any situation where concentration is needed.
- From dear Google
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Alright, the idiom is descriptive and clear, but
1. In English, not all sports use balls having ball-words in game names. eg. soccer, (table) tennis, golf, badminton, ... etc. And a badminton ball ain't called ball, but a shuttlecock, or a birdie casually (because it can fly in the air like a bird? 
)
2. Sometimes one has to hit the ball instead of keeping staring at it, like the poor kid below 
