The Union symbol (hieroglyph) is Gardiner sign listed no. F36, part of the series for parts of mammals. As a "union symbol", (a right and left half), it contains a vertical invisible 'centerline'. It allows for the positioning of two important hieroglyphs to be attached to it, right and left, as the uniting of two halves; specifically this is referencing Upper Egypt (by the King of the South), represented by the sedge hieroglyph (M23)
and Lower Egypt (the King of the North), represented by the papyrus clump hieroglyph (M16)
.
In Egyptian hieroglyphs, the hieroglyph is used for the phonetic value of sma, (a triliteral) with meanings of to join together, to unite with.