Basically just why do they move
The earth is made up of layers.
The top layer is the crust. It is solid and a bit brittle.
The next layer down is the mantle, which is also solid, but which is under immense pressure, and is also hot, which means that although it is solid, it can flow.
The mantle has convection currents flowing around in it, which are the same as winds in the air. The hotter material rises, and the cooler material sinks. Because it is solid, this happens a lot more slowly than it would in air.
The continents are a part of the crust, as are the sea beds, and because they rest on the mantle, they can’t all be joined solidly together, but are broken up into plates (tectonic plates). Each continent is part of one or more of these tectonic plates.
As material from the mantle rises under one edge of a plate, new rock forces its way to the surface, creating a line of volcanos. This tends to happen in the middle of oceans, such as in the centre of the Atlantic (although it also happens in other places too, like down the middle of Iceland).
The new rock pushes the tectonic plates either side of it apart.
Elsewhere, tectonic plates hit against other plates, and one slides down under another, also being pulled by sinking mantle underneath it. The crust melts and becomes part of the mantle, or may also rise up and form volcanoes, depending on how dense it is and the particular situation. This downward sinking of one edge of the plate pulls the rest of the plate with it.
So the mantle acts like a conveyor belt underneath each tectonic plate, towing it along, creating a new edge on one side, and eating it back down on the other side.
At the moment, it’s not clear which mechanism is more important: the tectonic plate sinking, which pulls the rest of the plate along, or the mantle pulling the plate along. It is probably a combination of both things.
In addition, tectonic plates can crash into one another, and both crumple up, to form enormous mountains, like they are currently doing in the Himalia.
Explanations are much, much better with some diagrams, so [Here is an excellent explanation for you](https://www.bbc.co.uk/bitesize/topics/zn476sg/articles/zrcgr2p).
There’s heat rising from below and liquid rock. It’s like soup warming on a cooktop; sprinkle something light on top and the currents of hot liquid rising, then cooling, and then sinking back down will move around anything floating on top.
– this should be tagged as planetary science or engineering or physics, basically anything other than “biology”.
and they move because there’s “liquid rocks” underneath them and they’re kind of “swimming” on those. basically they exchange heat and since nothing is perfectly even/homogenous this exchange causes stress which results in movement in one direction or the other.