The marker essentially announces that you are about to define or explain the noun to which it is attached, as the following example shows.
친구란 어려울 때 도와줄 수 있는 사람이에요.Dictionaries often show "as for" as an English equivalent of 란/이란, but normally we would not translate it in English. However, in spoken English, we usually show the emphatic nature of the marker by stressing the noun to which 란/이란 is attached and then pausing before continuing on with the definition or explanation of the noun, as demonstrated below:
A friend is someone who helps you in difficult times.
A friend--is someone who helps you in difficult times.In the above example, the boldface type is meant to indicate stress, and the hyphens (dash) are meant to indicate a pause.
Normally, Koreans also pause after the marker 란/이란 before continuing on with the definition or explanation of the word or phrase to which it is attached.