to rub your skin with your fingernails, often when your skin is itching. In the picture, the scientist is scratching his neck.
"She scratched her nose."
"My back is itching. Can you scratch it for me?"
From scratch (idiom)
If you do something from scratch, you do it from the beginning. When we start from scratch, we start from the beginning.
"I didn't give my daughter any help making this cake. She did it all from scratch."
"Bill Gates started Microsoft from scratch."
"I broke the pottery I was making. Now I have to start again from scratch."