Regelation is the melting and freezing of a substance due to changes in pressure rather than temperature.
Regelation, regelation has to do with changing the freezing point of a material. Remember that if we apply pressure to a material, that's going to reduce the amount of kinetic energy in that material so that if we have a material slightly above its freezing point but we apply pressure that's going to reduce that kinetic energy and cause it to freeze at a higher temperature. Conversely if we have less pressure, that's going to increase the kinetic energy in that material and that material will actually have to go to a lower temperature before we remove enough kinetic energy to cause it to freeze. Now that's true of almost all matter because almost all matter as it goes from liquid to solid contracts.
However, certain molecules like water expand when they freeze. So what happens to water if we apply pressure to it? Well we're actually going to force it out of its solid configuration into its liquid configuration which is more dense when we do that we're actually causing its freezing temperature to drop. So applying pressure to water actually lowers its freezing temperature. Now a good example of this is making a snow ball if you go out on a day that's below freezing pick up some snow if you apply pressure to that snow you can actually melt it enough so that it'll form a nice snow ball that'll stay together for your throw.
Now if it's really cold significantly lower than the freezing point of water no matter how hard you apply that pressure you won't be able to apply enough pressure to melt it to form a good snow ball. That's why really cold days are not good for snow ball fights, not only are you freezing but your snow ball is going to stay together. So that's little bit about regelation and how regelation can actually change the freezing point of water.