This quote, "Ignorance is Strength", is part of the larger slogan that Winston sees emblazoned on the pyramid of the Ministry of Truth. The complete slogan, which we need to examine to understand this part, reads as follows:
War is Peace
Freedom is Slavery
Ignorance is Strength
This quote, introduced so early on in the novel, is the reader's first introduction to doublethink. The constant bombardment of propaganda-based fear attacks the independence and strength of people's minds to so great an extent that they accept unquestionably facts that are blatantly the opposite of reality. For example, The Ministry of Love is really a torture centre, The Ministry of Peace is dedicated to prolonging war and the Ministry of Truth is commited to "altering" news and history books. These are facts which the people accept without challenge.
The slogan is thus an important part of this propaganda, and has its basis in a kind of truth that we the readers are aware of, and a truth that Winston comes to discover in the course of the novel. War is literally peace, because having a common enemy keeps the people of Oceania united together. Freedom is, indeed, slavery, because the man who is subject to the collective will is free from want. And lastly, Ignorance is Strength, for ignorance is what the power of the ruling party is based on.