hey the theory of plate techtonics is earth's crust and rigid upper mantle has broken into parts called plates over time. Its thought of to be the reason for earthquakes and volcanic eruptions. when a plate moves it can move away or push together. so say you have two pieces of paper that are plates the can either seperate from each other or come together and push on one another causing an earthquake in some places or an volcanoe eruption in others depending on where it is in the world. hope this helped and if u want any more help on the topic let me know:)
it is the study of earth's plates and litosphere. somethings to study in this area are how earthqukes form, volcanoes, faults, tsunamis. alll are relate to plate tectonics and the way the plates move