Push/Pull refers to the amount of time film is left in the developer when processing. This controls the amount of chemical reaction that can occur with the film emulsion, which controls your negatives density.
Push/Pull processing allows us to, in a sense, change the film’s sensitivity rating, or ISO, after shooting. This isn’t exactly what is happening, however it can be thought of in this way. If you only had 400 ISO film and you really needed 800 ISO film, you could shoot the entire roll as though it were 800 ISO and then have it pushed 1 stop (+1) in processing. The Push will generate more contrast and grain structure along with creating a shift in the color palette , but it will also compensate for the underexposure that takes place when rating 400 ISO film at 800 ISO.
Pull processing is the exact opposite of Pushing. If you only had 800 ISO film, yet wanted to shoot it at 400 ISO you can shoot the entire roll as if it were 400 ISO and have it pulled 1 stop (-1) to compensate for the overexposure.
Push/Pull processing creates a shift in the amount of contrast, the grain structure and the color palette in the film, but it works in a pinch if you have no other options.