When a solid is heated, it may melt and become a liquid. This is what happens to wax, for example. It is a reversible change. But many substances change before they get hot enough to melt. These are irreversible changes.
Moulding pottery clay then letting it dry is reversible: if the clay is wetted it can be remoulded. But firing clay in a very hot oven to make pottery is an example of an irreversible change caused by new substances forming in the clay.
