Symbols and collections support metadata, a map of data about the symbol or collection. The metadata system allows for arbitrary annotation of data. It is used to convey information to the compiler about types, but can also be used by application developers for many purposes, annotating data sources, policy etc.
An important thing to understand about metadata is that it is not considered to be part of the value of an object. As such, metadata does not impact equality (or hash codes). Two objects that differ only in metadata are equal.
That said, metadata and its relationship to an object is immutable - an object with different metadata is a different object.
(meta obj)
^obj
Returns the metadata of obj, returns nil if there is no metadata.
(with-meta obj map)
Returns an object of the same type and value as obj, with map as its metadata.