Safari Technology Preview was first released alongside OS X El Capitan 10.11.4. Safari Technology Preview releases include the latest version of WebKit, incorporating Web technologies to be incorporated in future stable releases of Safari, so that developers and users can install the Technology Preview release on a Mac, test those features, and provide feedback. 
For example, Apple released Safari Technology Preview 112 a few hours ago.
Apple has released Safari Technology Preview 113.
Apple has released Safari Technology Preview 114.
Apple has released Safari Technology Preview 115.
Apple has released Safari Technology Preview 116.
Apple has released Safari Technology Preview 117.
Apple has released Safari Technology Preview 118.
Apple has released Safari Technology Preview 119.
Apple has released Safari Technology Preview 120.
Apple has released Safari Technology Preview 121.