No, it doesn’t have anything to do with gymnastic tumbling. Tumblr is a blogging platform like WordPress or Blogger that allows its users to post and share content. It’s unique in the ability to “re-blog” content from other users and to curate a collage of articles, photos, and videos that all express a single user’s personal aesthetic. Teens love this because it allows them to express their individuality while interacting with a community of people who share the same aesthetics.
Who’s in this community? Anyone. Literally anyone. Tumblr is well known for requiring very little information from its users. All you need is an email address and a username/password to sign-up, and then you have access to almost every single thing ever posted to Tumblr. As a rule, “Tumbleblogs,” as they’re called, are public and cannot be made private unless you decide to set up a second blog. Individual posts can be made private before they’re published, but only if you select that setting. Otherwise, everything you post is available to every other user.