Understanding Rails (web App) User authentication


I am a seasoned network/security person and for the past month I have
been trying hard to learn and maybe more importantly understand web
application concepts.

I have been reading the purchased book Ruby on Rails Tutorial:Learn Web
Development with Rails 4th edition and overall I think I have been
soaking up most of the material however every time it seems when the
book talks about authentication using digests, tokens, etc. I find it
hard to fallow.

My question is can someone provide me a general writeup on web
application authentication specific to Rails hopefully in a similar way
as done in the book I am reading? Is there some place I go on the net
to find this where I can read/understand it? Any recommendations?

Thanks in advance,