For Winston Smith, freedom means being able to challenge the Party and its totalitarian control. In Oceania, the Party is all-powerful. It doesn't just rule the country with an iron fist; it tells people what they can do, where they can go, and even what they can think.
To secure absolute control over people's thoughts, the Party constructs its own parallel universe in which, according to the blatantly contradictory slogans put out by the Party, “War is Peace,” “Freedom is Slavery,” and “Ignorance is Strength.”
If anyone is to secure freedom from the Party's control, they need to be able to think for themselves. This means accepting that there's a world of fact where the Party's writ does not run. In this world, two plus two equals four. It always has and it always will, irrespective of what the Party says. But in the totalitarian dystopia of Oceania, if the Party says that two plus two equals five, then everyone is automatically expected to agree.
Winston understands that for people to have freedom, there needs to be a world of fact that is independent of what we or anyone else might think; a realm of truth that cannot be subverted by political or religious dogma. Mathematical equations form part of this world, and the ability to assert their absolute, unwavering truth, irrespective of what the Party might say, is an essential part of what it means to be free.