What does that mean?
It means that all visitors will be able to view your Blogspot domain blogs over an encrypted connection by visiting https://<your-blog>.blogspot.com. Existing links and bookmarks to your blogs will continue to work. As part of this change, the HTTPS Availability setting will go away, and your blogs will always have an HTTPS version.
Basically if you want to have visitors to access your blog over HTTPS, you can turn on HTTPS and HTTPS redirect for your blogger website.
HTTPS (Hypertext Transfer Protocol Secure) will allow you to set up an internet communication protocol that protects the integrity and confidentiality of data between the user's computer and the site.
This is great to help add security measures that make it harder for other people to listen to your visitors’ conversations, track their activities, or steal their information.