Plate tectonics is a hard term used to talk about a very difficult idea. For us on the Earth's surface, it is almost impossible to imagine that the Earth has a crust which is floating on material below (called the mantle) and that it is moving all the time. It doesn't move very much - just a few centimetres a year - so it is not normally noticeable. The crust is also split up into slabs, which scientists have called plates - and these plates move separately, sometimes pulling apart, sometimes sliding past one another, sometimes crashing together. So plate tectonics means 'crustal slab moving'.
All the same, it is still difficult to know what that is all about. It is less difficult if you live in a place near the edge of one of these slabs (plates). The slabs of crust are made of hard rock, so when slabs move past one another, or crush together, the rock does just bend, it stays where it is until the force is so great it snaps. So you get a long period when nothing appears to be happening (but forces are building up unseen) and then a sudden snap when the rock breaks. The sudden snap is an earthquake, and the broken line is called a fault. One of the world's best known faults is the San Andreas Fault that runs through San Francisco. California is littered with faults because it is near the edge of the slab of crust carrying North America.
The other thing you notice when slabs meet is that there are volcanoes. Why should that be? The answer is that the crushing of rock produces many snapped rocks, and liquid from inside the earth can use these lines of weakness to get to the surface. In the diagram on this page you can see an exaggerated idea of where these plates are. In real life there are no gaps between plates, but they are drawn wide here to make it easy to see the idea.
There is another idea called Continental Drift. It is connected to plate tectonics. You might say it is the result of plate tectonics. The Earth is very old, so the movements of the crust may be slow, but over millions of years it is very significant. It means that parts of the crust were not always where we see them today, but were once in other places, and that they have 'drifted' from where they were to where they are now, and in the future they will drift to new places.
Every time two pieces of crust drift against one another the rock in between is crushed. This forms lines of mountains as well as earthquakes and volcanoes. That is why mountains are in lines and why earthquakes and volcanoes are common in mountain areas.
