It is actually a ligature, i.e., a 'glyph' formed of two separate units called 'graphemes'. Grapheme is a fundamental unit of any written language; while a glyph could be the variant of graphemes, a compound character (ligature) or a diacritic (a character with marks present above or below it).
There is a short story behind the origin of this sign. During the period of the Tang Dynasty in ancient China, a young boy was on his way to appear for an examination to become a minister. The boy fell ill while he was traveling through a mountainous region. A herbal practitioner and his daughter took care of the boy, after which he recovered quickly.
When the boy had to leave, he found it difficult to say goodbye to the girl, as he fell in love with her. The girl thus wrote down the right hand part of an antithetical couplet for the boy; which was as follows. "Green trees against the sky in the spring rain while the sky set off the spring trees in the obscuration."
The young boy took it and left for the capital to attend the examination. During the examination, he was asked by the emperor to complete the right hand part of a couplet which said, "Red flowers dot the land in the breeze's chase; while the land colored up in red after the kiss".
The boy instantly realized that the sentence written by the herbal practitioner's daughter was a perfect match for the one given by the emperor. He gave the right answer and stood first in the examination. Impressed by his performance,, the emperor adjudged him as the new minister. After becoming minister, the young boy married the practitioner's daughter.
On the occasion of their wedding, the HAPPY character was doubled and put on a red paper. The symbol was used to mark the happiness of the event. Thus, it attained a respectable position in Chinese art of calligraphy and is used in weddings.
The various symbols of the Chinese language have a great significance and some kind of tradition associated with them. Chinese calligraphy is one such art that makes use of these characters. This symbol thus, is an important character, which is considered to bring positivity and happiness in the life of a newly wed couple.