Pancasila

Pancasila Link to a government page on Pancasila.

Here's another explanation from http://www.fotw.ca/flags/id-pent.html :

Pentagons commonly appear on Indonesian flags. The pentagon is a symbol of Pancasila, the national philosophy (in the past it was called national ideology), which consists of five principles. The word Pancasila itself means Five Principles/Five Bases Panca = five, sila = base/principle). It is not always a pentagon that is used to represent Pancasila's principles. In general, anything five-sided could be used to represent those principles. Another most common used emblem in Indonesian heraldry is a five-petalled jasmine flower, the national flower of Indonesia.
Sammy Kanadi, 13 September 2000

The Pancasila is also symbolised by the five parts of the coat-of-arms.

• star in the centre: belief in the one and only God (1st Sila)
• chain: just and civilized humanity (2nd Sila)
• banyan tree: the unity of Indonesia (3rd Sila)
• head of the bull: democracy guided by the inner wisdom of deliberations of representatives (4rd Sila)
• paddy and cotton ears: social justice for all the Indonesian people (5th Sila)

Source: Indonesia Handbook 1989

Some historical background :

.... the Indonesian state philosophy, formulated by the Indonesian nationalist leader Sukarno. It was first articulated on June 1, 1945, in a speech delivered by Sukarno to the preparatory committee for Indonesia's independence, which was sponsored by the Japanese during their World War II occupation. Sukarno argued that the future Indonesian state should be based on the Five Principles: Indonesian nationalism; internationalism, or humanism; consent, or democracy; social prosperity; and belief in one God. The statement was not well received by the Japanese authorities, but independence preparations for Indonesia were continued. Before Indonesia's independence was declared, however, the Japanese had surrendered and Britain had taken control of the country.

The Five Principles have since become the blueprint of the Indonesian nation. In the constitution of the Republic of Indonesia promulgated in 1945, the Five Principles were listed in a slightly different order and in different words: the belief in one God, just and civilized humanity, Indonesian unity, democracy under the wise guidance of representative consultations, and social justice for all the peoples of Indonesia.