How does word size affect the amount of virtual address space available?

Disclaimer: I'm not sure if this is the right site for this question or not, if it isn't, please let me know and flag to be moved. Thank you.

So, I should really know this stuff already, but I am starting to learn more about the lower levels of software development. I am currently reading Computer Systems: A Programmer's Perspective. by Bryant O'Hallaron.

I am on chapter 2 and he is talking about the way data is represented internally.

I am having trouble understanding something conceptually and I am sure that I'm about to make my ignorance lucid here.

I understand that a "word" is just a set of bytes and that the word size is just how many bits wide the system bus is. But, he also says: "the most important system parameter determined by the word size is the maximum size of the virtual address space. That is, for a machine with a w-bit word size, the virtual addresses can range from 0 to (2^w)-1, giving the program access to at most 2^w bytes"

I am both confused on the general relationship between the word size and the amount of addresses in the system and how the specific formula is w-bit word size=2^w bytes of memory available.

I am really scratching my head here, can some one help me out?