If you have men who will exclude any of God's creatures from the shelter of compassion and pity you will have men who will deal likewise with their fellow men. - St. Francis of Assisi
A monitor is like a speaker at the floor level pointed back to the musicians. They can vary in size, but typically have about a 2 x 3 foot footprint, and stand about 2 feet high. The drawback of monitors is that they block some of the view of the stage, and could often result in feedback from the microphones or speakers if they aren't placed properly or set correctly, and take up room when hauling the sound gear in the back of the station wagon! Back in the 90s some musicians I knew started to use personal monitors which were small electronic devices that attached to the microphone stand. Many musicians still use the stage monitors or personal monitors rather than the ear monitors.