tl;dr: it's my internal codename for a very ambitious project to fix all of the issues with primary and secondary blogs on tumblr. the project is not actually in flight right now, it's been on the shelf for years.
as you probably know, and lots of tumblr users know, primary and secondary blogs are pretty confusing. you can only like/reply as your primary, you can only make a secondary/sideblog private or have more than one member, but you can reblog and post and send messages as your secondaries, etc etc. the logic table of what you can do per blog on your account is weird, to say the least. we get feedback about this all the time.
this is a holdover from how secondary blogs were originally built: they were just tacked on to the existing user model, via your primary blog, at a time when tumblr was growing super fast. behind the scenes, your secondary blogs are the same data model as your primary blog, but the actual relationship they hold is with your primary blog, not with your account. so it's like your secondary blogs are "children" of your primary blog, rather than being siblings, as one would probably build it today if we could do it all over.
and there's a lot, lot, lot of code and plumbing that relies on this assumption, so it's not as simple as just fixing that problem and magically having the ability to like/reply/follow/etc as a sideblog. this stuff is very very old, from the time when tumblr was a blogging network first and not a social network, so it made sense then. it doesn't make sense today, from a product point of view, if you consider tumblr a social network first.
the most basic "fix" for this could be a simple account switcher, like other platforms have, but this wouldn't solve the problem for existing users who have tons of sideblogs they may want to "swap" their primary for, or reply/like as, or whatever. you'd have to make a whole new account, and/or we'd have to do some magic on our end to detach all of your sideblogs and make them into separate accounts for you, which would be pretty jarring.
personally i think that account switcher option is pretty lame: we can do better.
instead of an account switcher, i think it'd be much more interesting to be able to "act as" any of your blogs within the same interface, kind of like how fast reblogging lets you quickly choose which blog you're reblogging to. messaging lets you pick which blog you're sending as when you start a new conversation. why not have the same option when long-pressing on the like button? or the follow button? or the block button? there's some discoverability to figure out here for sure, but it's surmountable.
and this leads to some other interesting potential changes to how someone could use tumblr via multiple proper blogs, like having a different dashboard per blog. different set of likes per blog. a more straightforward way to break apart how you curate your experience on tumblr. easy to just throw away your old blog when you wanna restart, without having to make a whole new account. hence, you'd be able to have "identities", rather than a primary blog and sideblogs.
sounds neat, right? i think so. but it's a fair amount of work to get this done. a lot of untangling has to happen. it's totally doable, it's just tedious. some of the pieces are already in place on the backend, we just haven't picked up the rest of the work. it's hard to prioritize this because, while it sounds neat, the number of people who would actually use it is probably pretty small -- but we'd never know until we try.
anyway, that's identities on tumblr. ✨