In a sentence, it is a Monte-Carlo simulation of a simple model of ferromagnetic materials. Each triangle represents a single atom, which can have either of two spin states: up or down. In this simulation, red represents upward spin and black represents a downward spin. What these spins actually influence is the magnetic field produced by each atom. In turn, the atoms want to line up with the magnetic field, so each atom will try to get its neighbors to spin the same way it is. This influence is responsible for the formation of magnetic domains -- large regions of atoms lined up in the same way. However, there are thermal effects that can cause each atom to flip randomly. These thermal effects tend to reduce the size of the magnetic domains, and make the screen look more like a staticy tv.
This interplay between temperature and the interaction between atoms is actually the most interesting thing about this model (called the ising model). It is one of the simplest systems which exhibits phase transitions! That is to say, like fluids turning into gas, this system has a macroscopic property that does not change smoothly as we vary the temperature. In this case, it is the magnetizablility of the material -- how it reacts to an applied magnetic field. You may know that real ferromagnets have what is called a curie temperature, above which they cease to be ferro magnetic. This is exactly the phase change that is observed in this model. Below this curie temperature, large magnetic domains can form, producing a ferro magnetic effect, where small applied magnetic fields can create a large response, since there is a feedback loop where atoms aligning with the applied field encourage other atoms to do the same. However, when temperature is to high, there is too much randomness to preserve these domains. In this case, an applied magnetic field can no longer rely on the feedback effects, and so must try to keep the induvidual atoms aligned by itself, reducing the reaction of the material.
In this simulation, you can control the temperature by moving your cursor horizontally, and you can see the phase change by looking at the size of the magnetic domains. Play around with it!