What is the period you sometimes see at the end of a domain name?

The root node of DNS has a null label

The DNS tree is restricted to 127 levels of depth so you could only.have.a.domain.name.one.hundred.and.twenty.seven.levels.deep.com

. is used to mark a domain as absolute eg; utf9k.net.

Behind the scenes, a full domain name would be www.google.com.<root/null>

Some websites, or perhaps more accurately the load balancers and proxies in front of them, don’t acknowledge the existence of such a thing.

One high profile example is Amazon. If you visit https://amazon.com., you’ll see a blank page with the title x. Note the period on the end of the URL to see this issue in effect.