The letter 'G' refers to God or to a supreme being. A belief in a supreme being is a requirement for membership of a Freemasons' Lodge. This supreme being can be any one worshipped by an established religion. Hence Freemasonry is open to followers of any religion that has at its core the belief in a supreme being.
I hate to say this, but the fact yall say that the "G" stands for god is incorrect... The square is on target in the beginning, and the third symbol is the compass.. those are the three symbols that people would normally see.
The official symbol of Freemasonry is The Masonic Square Compass with a letter 'G' in the middle. The Masonic square and the compass are the principle symbols in Freemasonry. These symbols are always displayed in an open lodge. These symbols are essentially a lesson in conduct. It teaches that one must "square" their actions by virtue (square of virtue) and should "circumscribe" their desires. One should also bind their passion towards humankind. There is no universal interpretation for these symbols as Freemasonry is non-dogmatic. Since the symbols themselves do not denote meaning, these moral lessons are preached and communicated to the followers through the means of rituals.