Definition
In Korea, the Revised Romanization of Korean is the national standard rule for rewriting Korean sounds in the Roman alphabet, so that people who can't read Hangul can read and pronounce Korean words. It is used to keep Korean proper nouns and signs consistent, instead of translating them into English.
Why Romanization and Not English?
When you travel abroad and look at a local map, you can still ask for directions out loud if the names are written in the alphabet, even if you can't read the local script. Romanization works the same way. It's a bridge that helps foreigners who can't read Hangul read Korean place names and personal names just as they sound in Korean.
The key point is that this is not translation into English. For example, if you write 'Gyeongbokgung' (경복궁) only as its English meaning, 'Gyeongbok Palace', a foreign visitor who asks a taxi driver for directions in English may not be understood right away. That's why the Korean sound, 'Gyeongbokgung', is written out.
In the end, romanization is not a translation that carries meaning. It is an agreement for carrying Korean sounds into the alphabet the whole world shares. The rule is used on public guides like road signs and subway maps. Passport names follow it as a general principle, but exceptions are allowed.
It Writes the Real Sound, Not the Letter Shapes
The biggest principle of romanization is to follow the standard pronunciation as it is actually spoken, not the Hangul spelling. In Korean, sounds change smoothly when neighboring letters meet, and this happens very often.
For example, the Seoul place name 종로 (Jongno) would look like 'Jong-ro' if you copied the letters. But it is actually pronounced [종노] (jong-no). So the romanization follows the sound: 'Jongno'. In the same way, 신라 (Silla) is not written letter by letter. It keeps its real pronunciation [실라] (sil-la) and becomes 'Silla'.
Final consonants work the same way. The final consonant of 꽃 (kkot, 'flower') is ㅊ, but it is actually pronounced as its representative sound, [ㄷ] (the 't' sound). So when the word stands alone, it is written 'kkot', with a 't' that matches the real ending sound. The system is designed so that when a foreigner reads it aloud, Koreans can understand it easily.
A Bit More Precisely
To be more precise, the system in use today is the 'Romanization of Korean' (국어의 로마자 표기법) set in 2000. The earlier system used special marks such as the breve and the apostrophe (’), but they were awkward for typing on computers and searching online. So those marks were dropped, and the rule now says to avoid symbols other than the Roman letters wherever possible (a hyphen is used only when needed).
Even so, the rule of writing real sounds has a few clever exceptions. A hyphen (-) goes before the administrative units 도, 시, 군, 구, 읍, 면, 리, 동 (do, si, gun, gu, eup, myeon, ri, dong) and before 가 (ga), and sound changes on either side of the hyphen are not reflected in the spelling. Road-name addresses work the same way, with a hyphen before 대로, 로, 길 (daero, ro, gil). So the place or station name 종로 is 'Jongno', but the road name 종로 is written 'Jong-ro'. The point is to make the original form of administrative units and place names easy to recognize at a glance.
People's family names, company names, and long-established customary spellings also get wide room for personal choice. Historically settled spellings like 'Kim' (김) or 'Lee' (이) can stay as they are widely used, instead of the rule-based 'Gim' or 'I'.
Common misconceptions
Romanization is a set of rules for translating Hangul into English words.
It isn't translation. It's a rule for writing Korean pronunciation in the alphabet. That's why the principle is to write 한강 as 'Hangang', as it sounds, and not as 'Han River'.
Where you meet it
The nameplate at Line 1's 종로3가 station reads 'Jongno 3(sam)-ga'.
Korea's signature dish 비빔밥 is written 'Bibimbap' in the alphabet.
In one sentence
Romanization is a standard agreement that writes real Korean pronunciation in the alphabet, so foreigners can read Korean words aloud.