一目了然 yímù-liǎorán obvious at a glance (idiom)

怎么记住“一目了然”: 一 one + 目 eye + 了 particle: completed action / change of state + 然 however = obvious at a glance (idiom).

汉字

'Clear at a single glance' - instantly obvious, easy to take in - 用图表说明就一目了然 (yòng túbiǎo shuōmíng jiù yímù-liǎorán) shown in a chart, it's clear at a glance.
Praises clarity of a display, layout, chart, or explanation - the viewer grasps it immediately.
Contrast 一无所知 (utterly clueless); 一目了然 is about the thing being transparently clear, not the person's knowledge.