This is the 11173rd most frequent Russian word.
грань
Edge, facet, or boundary.
Here, "грань" is used metaphorically to describe the limit or boundary of acceptable behavior.
Ты пересёк грань допустимого поведения.
You crossed the line of acceptable behavior.
In this context, "грань" refers to the dividing line or boundary between two abstract concepts.
Грань между разумом и чувствами иногда бывает трудно определить.
The line between reason and feelings can sometimes be hard to determine.
Here, "грань" is used in a physical sense, referring to the edge or facet of an object.