Basic culture building of the Ancient Era. Available from the start of the game.
The Monument increases the Culture of a city, speeding the growth of the city's territory and the civilization's acquisition of Social Policies. It's the first building available for every city, without having to research any technology.
Note that under normal circumstances (that is, without any social policies or civilization modifiers) a settled city starts with no Culture, which means its borders won't grow AT ALL. If you want to achieve border growth for that city, you need to build a Monument first - hence the provision of one from Legalism, which saves a lot of time and production; or the opening Liberty bonus (+1 Culture per city). The Capital may gain Culture from Wonders, including the Palace as a national wonder - assuming completion of the primary Social Policy tree, it is permissible to sell the Monument midway through the game in order to temporarily save Gold.
The Monument is vital for continuing cultural buildings: in vanilla Civilization V it is required for the Temple (not to be confused with the Monastery luxury booster); in the expansions it is required for the Amphitheater. If one chooses to sell one's Monuments, they can be rebuilt after discovering Currency, when players will have more Gold to invest and will soon have access to game-changing Social Policies (such as those of the Rationalism tree).
A monument is a structure built to commemorate an important person, event, deity, or concept. The more important monuments are usually constructed near the center of the city, by the ruler's palace, or near the city's main gates. Monuments come in all shapes and sizes, from the Great Sphinx of Giza, Egypt, to the Statue of Liberty in New York City, USA, to Nelson's Column in London, England. The best of them imbue the city's inhabitants with a great feeling of civic and national pride.