Growth on trunk
I thought at first that this fungal fruiting body was a Hericium, a tooth fungus. However, looking at the picture enlarged, it does not look like there are little teeth like structure on the lower surface. It is a bracket fungus of some sort, but I can't tell for sure from the picture. If is is in a location that you can go back to and take a picture of it with further development, I would be interested. Many of these fungi are wood decomposers. It may be a signal that the tree has some heart rot and is in decline. If you know what type of tree it is, that may help too, because sometimes these fungi are specific in the types of trees they are associated with.
Thank you for contacting Cooperative Extension,