Before revision: This leaves open the question of what is 'natural' (in physicalism 'natural' means procedural, causally coherent or all effects have particular causes regardless of human knowledge and interpretation and it also means 'ontological reality' and not just an hypothesis or a calculational technique), but one common understanding of the claim is that everything in the world is ultimately explicable in the terms of physics.
After revision: This leaves open the question of what is 'natural' (in physicalism 'natural' meas procedural, causally coherent or all effects have particular causes regardless of human knowledge and interpretation and it also means 'ontological reality' and not just an hypothesis or a calculational technique), but one common understanding of the claim is that everything in the world is ultimately explicable in the terms of physics.