The metal marker from the Trinity Church graveyard at New York marks the grave of a Mason. It is the metal reproduction of what is perhaps the most recognizable emblem of the Freemasons, the square and two compasses. In this example the letter “G” appears in the middle of the emblem. Often the emblem is seen without the letter “G”.
Each component of the symbol represents a different Masonic orthodoxy, though, these are not hard and fast:
The compasses represent the boundaries of wisdom a person should have the strength to circumscribe and stay within.
The square symbolizes virtue in all actions, just as the expression “square deal” means treating people with fairness.
The letter “G” seems to have more than one meaning. It could possibly mean God, as in the creator of the universe; or Gimel, which is the word for the third letter of many Semitic languages. The number three is significant to many Masonic rituals and beliefs. Some also believe the “G” may represent geometry.
Gravely Speaking 
Always interesting background… your search machines must get a good workout! The first letter of many of the Semitic languages is “aleph” א … it is the tiny tattoo on the heal of my right hand!
Although this question is very general, it can be presumed that the question is referring to the most publicly-viewed symbol of Freemasonry: the Square and Compasses. These two symbols have deep symbolic meaning for Freemasons, but the beginning of understanding them is to compare them with tools used by operative masons.The Square is so named because it is operatively used to test objects for being “square”, that is, a perfect 90-degrees in angle. A Square can also be used to create other angles by employing trigonometric measures to mark out a desired angle. To a Freemason, one of the primary meanings of the Square represents virtue. A common colloquialism with Masonic origin in American English is “square deal” which means an equitable exchange of monies, services or goods, meaning that both parties have acted with virtue.The Compasses (note, this is a plural word – a “compass” is an instrument used to determine magnetic north) are operatively used to scribe circles. These are necessary to create perfectly round objects. To a Freemason, one of the primary meanings of the Compasses is to keep one’s passions within limitations, so as not to be controlled by them.Frequently one might see the letter “G” enclosed within the Square and Compasses. One of the primary Freemasonic meanings of this letter is that it represents “Geometry” or “God”.