If you look at a modern drum kit - (formerly known as a "trap set" - snare drum = trap, get it?) - you will notice, (usually, drummers are weird), a number of cymbals, including a special stand with two cymbals facing each other controlled by a foot pedal - (formerly a "sock cymbal", know known as a "high hat"), about a 14" drum, often with a metal body, mounted flat in front of the drummer - (the aformentioned "snare drum"), one or two big round drums mounted vertically right in front of the drummer and with mechanical pedals to strike them - these are the "kick drums", usually one or more large tom-toms mounted like the snare, and next to it - these are called "floor toms", and an array of smaller toms, cow bells, other drums like timbales, chimes, gongs, etc, etc, etc. When a 4/4 beat is played with each primary beat on the kick drums (which are on the "floor", get it?) it got the slang name "four on the floor", kinda disparagingly. For an example we've all heard, think Rod Stewart's "Da Ya Think I'm Sexy"....
*whump, whump, whump, whump* repeat as needed.