"Church Militant" refers to the Church on earth, which struggles against the forces of evil while working for the transformation of the world through the Gospel and good works. It is part of the larger Mystical Body of Christ, along with the Church Triumphant in heaven and the Church Suffering in purgatory
Ephesians 6:12 (New International Version): "For our struggle is not against flesh and blood, but against the rulers, against the authorities, against the powers of this dark world and against the spiritual forces of evil in the heavenly realms."