A water molecule is an oxygen atom bound to two hydrogen atoms. An understanding of these atoms is all that is required to understand all of their interactions with any other configuration of atoms. Molecules are an abstraction. But then again, so are atoms.
A bicycle is a thing that gets you from place to place, faster and with less effort than walking. But this is an abstraction: the tires can go flat, there are hills, it is raining.
A game of chess is an abstraction of a bunch of pieces on a board. When you flip the board over and scatter the pieces, the abstraction leaks.
An abstraction is a thingness that is laid over the parts which make it up. Sometimes a change of context affects the thing. Sometimes the partsiness of the thing leaks through. Understanding the parts can help you understand the thing. But often, looking at the thing as a thing gives you other insights into its nature. Many perspectives are required for deep understanding.
God is an abstraction of humanity.