A phase transition is where heating or cooling results in a substance reordering its component atoms into a different structure, thereby changing its density, volume, and other properties. It differs from thermal decomposition and chemical reactions in as much as nothing is emitted and no changes in chemical composition occur.
If the transition just results in the angles and lengths of chemical bonds changing, e.g. as denoted in changes between low and high forms, then this happens virtually instantaneously as the temperature goes over the threshold. If it requires bonds to be broken and reformed, for example in transition between silica and cristobalite, then this is a much more gradual, slow process.
Silica is the best known example to potters of something that undergoes phase changes, but this also occurs with other materials used by potters, e.g. boron and manganese.
Note that potters have followed the convention used by geologists, calling the low temperature form of a substance 𝛼 and the high temperature form 𝛽. Physicists, on the other hand, call the low temperature form 𝛽 and the high temperature form 𝛼. To avoid the inevitable confusion, the terms low and high are now used by all parties.