North Korea's flag, the Ramhongsaek Konghwagukgi, was adopted on September 8, 1948, in the same month the Democratic People's Republic of Korea was established in the northern half of a Korea divided after the Second World War. Kim Il-sung guided the design, which had to mark out the new state's politics and distinguish it from the flag flying in the south. It has not been altered since.
Reading the flag
A wide red panel runs across the middle, flanked by blue stripes and separated from them by thin white fimbriations. Set in the red panel is a white circle holding a red five-pointed star.
The star is communism and the revolutionary tradition the state claims for itself. Red is the bravery and sacrifice of the Korean people in pursuit of a socialist future. The blue stripes stand for sovereignty, peace, and friendship, the vocabulary the DPRK uses for its foreign relations. The white fimbriations are given as purity and national unity.
Flying it
The flag is displayed at government buildings, military facilities, and major public institutions, and at state ceremonies and diplomatic events. Rules governing its size and manner of display are strict. Ceremonial and military versions add further insignia.
Inside the country the flag is bound up with Juche, the state ideology of self-reliance, and it is a standard element of the public celebrations through which that ideology is rehearsed.
Family resemblances
The red star ties the flag to Soviet symbolism and to socialist flags more generally, where red and stars were standard. The particular arrangement of panels and fimbriations, though, is not easily confused with anything else.
A note on the name
Ramhongsaek Konghwagukgi translates roughly as "red-blue republic flag". Fimbriation is the vexillological term for those thin white stripes between the red and the blue.
References
"National Symbols of the Democratic People's Republic of Korea" via Official Website of the DPRK