MECE means “mutually exclusive, collectively exhaustive”, and is a framework to describe a whole by listing its parts in a methodical way. That seems somewhat dry and useless, but you cannot understand something unless you can describe it, and other than MECE I have never heard of any other way to methodically start to describe things (or even talk about the philosophy of describing as a general concept (except in the via postiva / via negativa way)).
So, MECE. You describe a thing by creating a list of things, and the totality of the things fully covers all aspects of the thing (collectively exhaustive), and none of the things overlap. Learning the MECE skill has been and continues to be incredibly valuable to me personally.
As a simple example, in the context of physics, force = mass x acceleration (Newton’s second law). F = m · a is collectively exhaustive because the equation for force has exactly two components, and because mass and acceleration are independent variables then the f = m · a description is also mutually exclusive.
There are lots of ways to be MECE, and the trick is to be insightful about the description.