Timesheets enables you to track an attendee's time in a session, most often used for tracking education credits.
Since you must use Breakout Sessions in order to track time you'll need to be familiar with building an Agenda in Connect Space. If not, check out How to Build Your Agenda before continuing.
You also must know how to check an Attendee into a Breakout Session, check out Breakout Session Check-in and Check-Out.
Simply put, time-tracking starts once you check an Attendee into a Breakout Session. In order to stop tracking their time you must be sure to check them out of the Breakout Session as well.