What does 靑天白日滿地紅旗 mean?

靑天 (청천) means "blue (靑) sky (天)
白日 (백일) means "white (白) sun (日)"
滿地 (만지) means "the whole (滿) ground (地)

紅旗 (홍기) means "red (紅) flag (旗)

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If we start with "flag" (旗) as the head noun of a Chinese-style relative clause, 靑天白日滿地紅旗 translates as follows:

"a flag (旗) [with] a blue (靑) sky (天) [and] a white (白) sun (日) [on] a background (滿地) that is red (紅)."

So, 靑天白日滿地紅旗 is the descriptive name for the flag of the Republic of China: