"Popping" is the unwanted noise which sometimes occurs when the subject makes a sudden forceful sound such as "P". It happens because this particular vocalisation forces a large amount of air into the microphone. Obviously it is more of a problem when the subject is very close to the mic.
One solution is to place a pop guard (AKA pop filter) between the subject and the mic. This acts as a barrier and absorbs most of the excess air pressure.
You can make an effective pop guard out of a wire circle and nylon.