@pollito,
It's brand new and still at beta (or maybe even alpha) stage. You can read more about its progress
here. It is, indeed, calibre's move to providing a fully integrated book editor.
To get access to it via the standard calibre GUI, for the time being, you need to do more than 'select book & press T'. First you will need to go to
Prefs - Tweaks - Plugin tweaks, change the relevant option to
test_tweak_book = True then restart calibre. Once you've done this 'select book & press T' will give you the new beta Ebook Tweak rather than the standard tweaker.
Alternatively you can use
ebook-tweak.exe like any other Windows program to open epubs (and azw3s???) outside the calibre GUI, just like you can use ebook-viewer.exe to read epubs etc.