/sci/ - Science & Math

I guess this is as good of a place as any to ask:

I'm reading Computer Systems A Programmers Perspective on my own, and am stuck on pic related.

I don't understand mechanically how they're using these summations.
For the practice problem Hex: A, in binary is 1010.
w = 4 so I sum up to 3.
What is x subscript i? Is the bits?

0(2^0)+1(2^1)+0(2^2)+1(2^3) = 8 ?
What is this supposed to mean?