All matter in the world can be broken down to the same essential parts. Grisha have the ability to manipulate matter at this fundamental level, giving the appearance of magic to what is actually a precise, deliberate science that abides by physical laws, as must anything else in nature.
As an example, an Inferni does not create fire from nothing, but rather summons combustible elements from the air; she can summon existing matter, but still requires a catalyst (i.e. heat) to start the chain reaction that will produce a flame. Thus Inferni will commonly carry a flint that can be used to ignite a spark.
A guiding principle in Grisha theory is that "like calls to like," meaning that some quality in the Grisha gives them an affinity for a certain kind of matter. That quality attracts them to other things sharing that quality. Ravkan philosophy uses the terms odinakovost ("thisness") and etovost ("thatness") to further explain this phenomenon. Odinakovost refers to the essence that all things share, while etovost refers to what makes a thing unique, and unlike anything else. Grisha must understand both in order to manipulate matter, but it is their command of etovost in particular that sets Grisha apart from non-Grisha.