All of Tumblr is served over HTTPS by default.
HTTPS is used to create an encrypted link between a browser and web server. This prevents eavesdroppers from snooping on the traffic between the two. Think of it as a tasty tortilla keeping the inner ingredients of your burrito a mystery. Is it chicken or three bean? Only the chef and customer can know for sure.
HTTPS and themes
HTTPS and custom domains
We support HTTPS for blogs with custom domains. After you add a custom domain name for your blog, it will take a little while (typically less than a day) for it to activate. You'll receive an email when it's ready.