The Resistance consists of the only free humans on Earth, living on the last human city of Zion for warmth, power and protection. They also know the truth about the Matrix and some of them are Redpills.
According to their tradition, The One freed the first of them nearly a century before and were taught the truth about the Matrix, founding therefore Zion. However this had happened 5 times before.
Resistance had various roles in the war with the Machines. Some of them sabotaged or attacked Machines, while others found potentials to rescue from the Matrix and take them to their cause.
The free humans learn that The One would return, in fulfillment of the Prophecy, to end the war once and for all.