It means something is very clear and easy to understand at a glance.
目看；了然清楚，明白。一眼就看得很清楚。
明·张岱《皇华考序》可见按图索籍，三溪道路，一目了然。”
The diagram made the entire process clear at a glance.
Learn the next connected idiom: 一目了然 (yī mù liǎo rán) → 燃眉之急 (rán méi zhī jí)
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