The concept of "2+2=5" in 1984 can be a bit confusing at first. This is because it is a statement from the book's made-up Ingsoc ideology. Basically, the statement represents the way that the Party controls the minds of the citizens of Oceania.

Consider how it is commonly accepted in contemporary society that 2+2=4. This is known as an objective truth to us, based upon our universally accepted understanding of what math is and how math works. But in 1984, the Party teaches children that 2+2 actually makes 5. This is an example of the way that the Party manipulates people on a deep psychological level. The Party made up this statement and others and brainwashes people into accepting them as objective truths. In doing so, the Party is able to maintain total social control.

Before Winston is tortured in the Ministry of Love, he understands that the notion of 2+2=5 is a mechanism of the Party's universal control. But recall how in the end, he traces 2+2=5 in the dust on a table. This represents that despite all of his efforts to resist the Party's influence, the Party still ends up breaking his soul and controlling his mind.