The tweet holds a link to an article in the NYTimes -- which is behind a subscription paywall.
"The creator of the mysterious CIA sculpture which has obsessed cryptography fans for decades has revealed another clue."

But I've found another source -- it's a 4-minute audio clip. "NPR's Mary Louise Kelly speaks with Jim Sanborn, creator of a cryptographic puzzle sculpture called KRYPTOS located at CIA headquarters, about his decision to release a third and final clue."

More digging, more searching, more links to more articles can be found. Here's one:

The first clue, which was released in 2010, was the word 'Berlin' from letters 64 to 69.
The second, released in 2014, was the word 'clock' from 69 to 74.
The third and final clue that he recently announced was the word 'northeast' found from letters 26 to 34.

The assortment of letters now read: