INSERT ... ON CONFLICT inserts a row or, if it violates a uniqueness constraint, does nothing (DO NOTHING) or updates the existing row (DO UPDATE). It is an atomic "upsert", safe under concurrent access. EXCLUDED refers to the values you were trying to insert.
Example
INSERT INTO stock (sku, qte) VALUES ('A1', 5)
ON CONFLICT (sku) DO UPDATE SET qte = stock.qte + EXCLUDED.qte;