Lens was announced at Google I/O 2017, and began rolling out to Photos a few weeks ago. If you're not aware, Lens is a machine-learning powered spiritual successor to Google Goggles, as it can identify objects and landmarks in photos. Google said it would eventually arrive in Assistant, and now it looks like the time has come.
Some users are seeing a new Lens button in Google Assistant, and tapping on it opens the camera. From there, you just tap on the object you want to search, and Assistant will try to figure out what it is. It's pretty much the same as Lens in Google Photos, except you don't have to take the picture before-hand.