If your grandmother's handwritten pie recipe calls for "some sugar," "plenty of butter," and "sliced apples," you might ask her to expound by adding measurements of how much of each to use. When you expound, you explain or provide details.
Expound came into English from a 14th-century French word espondre meaning "to elaborate" or "put forth." Often when you expound something you are clarifying or giving the particulars. A more formal way to expound is to offer exposition, often going line by line to explain precise meanings in a text or legal document.