A paradox is a statement that contradicts itself but can possibly be true (or one that seems at first to be true but isn't).
The three party slogans on the side of the building are
War is Peace
Freedom is Slavery
Ignorance is Strength
These are clearly paradoxes. It is easy to see how these statements contradict themselves. War and peace, freedom and slavery are opposites -- not the same things like the slogans say they are.
But could they be true? In the society of 1984 Oceania, you could argue that they are. War brings peace for the party at least. Ignorance could be strength from the point of view of someone in that society since ignorance can insure that you don't get punished.