Enfranchise means to give someone the right to vote. Disenfranchise means to take it away. The U.S. has a shameful history of disenfranchising African-American citizens through bogus laws and outright intimidation.
Being disenfranchised can make you feel like you don't belong or that you have no power. When kids feel disenfranchised in school––when they feel like what they're being asked to learn doesn't matter, or that their teachers don't care about them, that whether they try or not they're going to fail––they stop learning.