If your grandmother says you look like skin and bones, it means she thinks you're too skinny.
Use the term skin and bones when you're talking about someone who is very thin. It's not a compliment; if a person is skin and bones, your usual reaction is going to be wanting to feed him a huge meal. You can use skin and bones in a lighthearted way — "Look at you, honey, you're all skin and bones!" — or in a more serious way, when you're referring to a truly malnourished person.