If you select the "public" access level when posting content on your site (e.g. a blog posting), this content will be accessible to visitors of your site also when not logged in.

If you go to the Advanced settings page in the admin area of your site, there's the "default access permission" option. If you select "public" here, the pre-selected access level for all users will be public from now on (when posting new content without selecting another access level for the post itself).