What is TK?
It’s a publishing tradition where, during writing, the author or editor writes TK in various places in a story as placeholders or reminders to come back to it later. For example:
He reached out to John Lastname TK.
The value of the stock options topped $300,000 TK VERIFY
The origin of TK is murky, but it is believed to be a version of to come, intentionally spelled out in a different way and capitalized so it is easier to spot and impossible to appear organically in the writing.
Before publishing, the editor looks over the article to make sure no TKs are left. (Although, sometimes, they miss some.)
Just like many before you, you can use TK to leave yourself reminders for later. Medium will highlight TKs in your story and warn you before publishing if any remain.