Molds are data types in Hoon. Atom auras like @ud are molds, as are more complex structures like json, path, (list @ud), (unit manx), card:agent:gall, (map @tas (pair @t @ux)), etc. Raw values may be type-cast to a mold, and molds can also be called as functions to coerce a value to its type (this is called "molding" or "clamming"). Molds may be create with "mold builder" functions, or with $-family runes.
Further Reading
Hoon school: types: A lesson on molds.
Buc (
$) rune reference: Details on the runes that can create molds.