To co-browse is to follow someone else's browsing session in a document preview. Multiple people can co-browse the same file (see What is document co-browsing?).
The selected profile picture gets bigger and moves to the front of the row to show it is selected, and your view of the document preview mirrors that person's view.
To return to your own view, click the enlarged profile picture, and from the resulting menu, select Stop Browsing With.