Realism means to depict objective reality without embellishment or interpretation, implying that a person (here the artist) can perceive the world in such a way and stay completely undistorted by personal bias. It is based on the belief that truth can be discovered through senses.
Realism as an artistic movement developed in France in the 1850s as an opposition to prevailing Romanticism. It revolted against the exaggerated emotionalism. Yet, it comes up as a label for several art movements or groups during art history as well.
Most commonly realistic paintings depict people at work, everyday situations, scenes and objects in a true to life manner, sometimes stressing the ugly or sordid of the true life.