This is a really intriguing English phrase that likely sounds rather ridiculous when it's first heard. The phrase is actually based on driving an automobile. The concept is that of pushing the gas pedal in an automobile to the metal floor (under the carpet), causing the automobile to go as fast as possible.
The phrase is used when a person may ask another person to hurry up.
“Put the pedal to the metal and finish packing, we have to get to the airport!” It is not a very polite way of asking and not highly recommended to use, but it is good to understand if a person says it. Another variation is to say “Floor it“, again a reference to the same activity while driving an automobile.