That's the term for secret bases that are set up so that you can fight the trainer with 3 level 100 Blisseys, the Pokemon with the highest experience yield possible. If the bases are set up correctly, the Blisseys only know Healing Wish, meaning the first two will knock themselves out, leaving you to take out the third.
Here's how to set it up:
Find Blissey Base guides on the internet. This Reddit Guide is one of the more popular ones.
Go into your personal secret base, and interact with the PC
Go to Manage QR Code patterns->Find a Secret Base
Scan QR Codes of the Blissey bases.
Most bases are set up in Secret Meadow or on the Secret Shore due to the many secret base locations in close proximity to one another, and the fact that these locations can be flown to with ease. You will need to use the HM Dive initially to gain access to these areas.
The bases' trainers only become interactable from midnight the day after you scan the QR codes to interact with the secret base trainers.
You can only battle each trainer once a day.
Use the Level Release object in the base to make the Blissey level 100.* *You must have obtained the level release object yourself to activate it.
(It is at the Battle Resort - in the homes to the far right of the Battle Maison past the bridge. You will have to complete the Delta Episode to access this area). Watch this video for the exact location
If the secret base is already occupied by another user, you can delete that user from your PC in your secret base.
Only the base owner will have Blisseys - the other trainers in a base will have randomly-generated Pokemon based on their trainer sprite/avatar (i.e. different Pokemon if they are a Ninja Boy vs an Ace Trainer).
Probably the best way: Having friends with secret bases. The fights there grant you a insane amount of XP if he uses the "right" pokemon (mainly blissey is used).
If you don't have any friends which have the game, there is only the old option to grind the top 4. Don't forget that they level up to level 70 if you rechallenge them, so it is almost as good as le wow in X/Y.
Having a lucky egg makes exp farming much easier since it grants you more exp (as much as if you traded it with someone). You can get lucky eggs from wild pelippers anywhere.
I usually give my Pokemon the Lucky Egg item and use one stronger Pokemon to plow through the Elite Four. If you give your leading Pokemon an Amulet Coin, then you can also make a lot of money after each battle.