By core loops, we mean the loop of basic actions the player needs to perform in a game.
As we learned, these actions are a set of core mechanics that the player has to interact with. The loop happens when the player interacts with the mechanics over and over again in order to progress into the game.
In fact, some designers have defined core mechanics exactly like that: The core mechanics in a game will usually be the purposeful interaction that occurs the most frequently. In a platforming game, this is usually jumping. In a shooter, it is usually shooting. In a racing game, it will be driving.
There could be other definitions, as with every other game design concept. As we have said many times already, you don't have to stick with ...
