A Core Team is when you take two pokemon that can cover each other weaknesses and build a team around those two. For Example Gliscor has pretty good Defense and is a flying types so it can take Ground types attack. You can pair him with Tentacruel who has Good Special Defense and can take Ice Type attacks for Gliscor. They take care of weaknesses for each other.
+ = Tentascor a OU core match up. I can give more examples if you want.