Briefing and Debriefing, why is so Important 

Every time you do anything, from BJJ to learning how to play the guitar, you must have a clear idea and a plan of execution of what you want to accomplish in your training session; you must discuss with your trainer/coach how and what; that is Briefing.

After you finish your first session, ask your coach or trainer for a debrief, and ask him; what you did well, what you need wrong and how we will fix it right now. 
The mantra is: 
Debrief and do it again; debrief and do it again.