It depends on the context but briefly/generally:
Push: Underexposure combined with overdevelopment. This is done either for aesthetic effect or because there is not enough light to give full exposure based on the film speed. So overdevelopment is used to squeeze slightly more shadow detail/contrast out of the film.
Pull: A little less clear depending on the situation. But generally it refers to reduced development to reduce contrast. This is often combined with increased exposure to give more full/open shadow detail (which can be lost when development is reduced).