Glazes are stocks so reduced that two litres of stock makes only 100 ml glaze. At this level of reduction it is very easy to burn the glaze, so if you notice it sticking to the bottom of the pot, complete the reduction using a bain-marie.
Glazes will keep in the refrigerator indefinitely and are useful when used in minute amounts to add flavour and gelatine to sauces in need of these qualities.