Originality Reports in Google Classroom will let students and teachers check documents for
Originality Reports is a feature that is in a beta testing period. G Suite for Education domain administrators can apply here to have their schools participate in the beta.
I've applied to participate in the Originality Reports beta on behalf of two domains that I manage. Until then all I can tell you about the Originality Reports functions is what I've gleaned from Google's announcement and GIF of the feature. It appears that Originality Reports will display in the margins of documents any possible matches for sentences or phrases found online.
From a business standpoint, Originality Reports certainly looks like an attempt by Google to compete with other plagiarism detection services like TurnItIn.
On a related note, in a few weeks I am releasing an on-demand course that will be full of practical ways to use all aspects of G Suite for Education in your classroom. Register here to be notified when the course is available.