To adumbrate something is to outline it. In an English essay, you could adumbrate the themes in a novel; or, in a letter to Santa, you could adumbrate all the ways you have been behaving.
Adumbrate is built on the Latin root umbra, "shade," and the image it evokes is of a shadow being cast around something. Your outline is like a shadow of something bigger — like the themes in that novel or the ways you have been behaving. You can also use adumbrate to mean "foreshadow": "The scene where the princess dreams of the vampire adumbrates her later discovery that her little brother is, in fact, a vampire."