Mathmatics is normally considered the "purest" type of scholarship, since it is entirely abstract. It would thus seem to be the least likely to be subject to political manipulation, the "final frontier" where everyone can agree on absolute truths. When the Party insists that it has the right to declare that 2 + 2 = 5, it is making a claim to total control over every aspect of objective reality. Nothing is beyond its fiat. You will remember that O'Brien also claims to Smith that the Party could cancel the laws of gravity if it should wish. Forcing Smith to assent to the proposition that 2 + 2 = 5 indicates that the Party demands to be in control of every aspect of his thinking and every framework through which he constructs reality. The Party's will is all; "objective reality" only exists insofar as it conforms to the Party's program.
In the nation of Oceania, the Party controls all aspects of life, including thought, this is particulary evident in the process of Doublethink. In order to control the minds of the members of Oceania, the Party engages in Doublethink, which engages the mind into holding two contradictory ideas in the mind at once. It occupies the mind so as not to allow independent thought. Through total psychological manipulation, the Party is able to make individuals believe anything that it desires. Such as 2+2 = 5.
Therefore, when the Party tells you to believe that 2+2=5 they are in fact shaping reality and have total control of existence.
It shows how the most basic and simplist thing like math could be over powered by the will of the government. The government had enough power to influence to taint even the purest thing like math. It shows how much control the government had over the people and knowledge that the people had.
It just shows how much power the government had over the people mathematically the correct answer is 4 but if the government said that the answer to that is 5 then it is 5. You cannot ask any questions people blindly followed the government orders and anyone who defied them went through agonizing pain until they wholeheartedly believed in what the government said.