That's by value. I reckon since Chinese imported goods tend to be cheaper they probably make up a greater number than the value percentage would indicate.

So I suspect 40% figure is how many items in a typical household are from China.

Edit: in case that's not clear, here's an example:

I have one item from the USA that costs $80. I have four items from China that cost $5 each.

My imports from China are 20% of my spending. My imports from China are 80% of my goods.