What is back focus ?

It's when your lens is slightly out of adjustment with your sensor.  Your images will be consistently focused behind or in front of whatever it is you are aiming at.

Lots of information on the net describing this and information with how to deal with it.  Look up "Len Align".

Your camera may have a micro AF adjustment to correct this.  It will be described in your manual if it does.  Your manual will also have a section that describes under what conditions your AF can be fooled.  Don't mistake this for a back/front focus problem.