Syneresis is the expulsion of liquid from a gel or network. Whey appearing on yogurt and liquid weeping from a set dessert are familiar examples. Changes in temperature, acidity or the structure of the mixture can affect how much liquid separates.
It is different from emulsion separation, although both can appear as weeping. The appearance of liquid alone does not identify spoilage or prove safety; storage history and the particular food still matter.
Reference: Further reading.