What is the exclamation mark for after the variable?

Question

I'm trying to understand the snipppet from graph ql appollo tutorial. What is the exclamation mark for after the variable? ID!

  query GetLaunchById($id: ID!) {
      launch(id: $id) {
        id
        rocket {
          id
          type
        }
      }
    }

Answer 1

It means it's non-nullable.

See : https://graphql.org/learn/schema/