The Web of Fate.
If Lake was told about this before he went to the Simplistic Realm, he really wouldn't be able to understand it.
Its complexity was both simple yet endlessly complicated to the point where Lake wasn't even sure that he understood it despite being able to make use of it.
Simply put, the Web of Fate was a sort of subspace that existed parallel to the world. In this space, an endless amount of fates were connected to each other to form a web-like structure. These connected fates show the connection between two or more peoples' fate and allowed Lake to see the histories, interactions, and even futures of that these people held in relation to each other.
Just now, Lake used the Web of Fate to track down Evelyn's and Leah's fate by observing Landur's fate to the point in history where the two were near each other before jumping over to Evelyn's fate.