To overshadow is to appear more important or larger than something else. Your sister's tendency to interrupt and speak loudly sometimes overshadows what a caring person she is.
You can use this verb in a very literal way, to mean "cast a shadow over something smaller," like the enormous oak tree in your yard that overshadows your small house. It's also possible to figuratively overshadow, like when your worry about next week's math test overshadows the happiness you feel waking up to a snow day. One person can overshadow another too, when they appear more interesting, successful, or attractive.