DUAL is a dummy table provided by Oracle for using when evaluating expressions which don't actually need to reference a database object, e.g.
SELECT 1 FROM DUAL
PostgreSQL does not provide an equivalent as it is capable of evaluating expressions without referencing a database object, e.g.
orafce implements a
DUAL table to assist with migrating from oracle, created as:
CREATE VIEW public.dual AS SELECT 'X'::varchar AS dummy;
Implicit implementation in PostgreSQL
In 2018, Tom Lane suggested a potential use case where the planner could benefit from inserting a dummy table into the planner/rewriter which would simplify various code paths: "SELECT ... FROM DUAL" is not quite as silly as it appears. This was subsequently implemented as commit 4be058fe in PostgreSQL 12.