What is the meaning of -은?

The Korean topic marker, with various nuances according to context:

1. Used to mark an already known topic, to which the subsequent statement applies.

   Used to mark an already known topic, to which the subsequent statement applies.

   Used to mark the topic in statements of general fact.

   what about; Used without a predicate to demand new information about an already known topic.

   Used in contrastive constructions, often with an exclusive sense (i.e. this and not anything else).

   Used with an emphatic sense.

   Used to background previously known information, in order to highlight the importance of the statement which follows; the marked topic is omittable.

   that, which; past adnominal suffix for verbal stems, often carries a perfect meaning.

   Realis adnominal suffix for adjectival stems.

   Source: wiktionary.org