The ancient philosophers developed a number of tools through which they could understand reality. One of them, from the writings of Aristotle, the student of Plato (who was himself the student of Socrates), is called the "Four Causes." The term "cause" is a little misleading. These four causes were the way you explained or defined something.
Today we think of a cause as something that answers the question "what brought a thing about?" But that is only one of Aristotle's four. It is called Efficient Cause. For Aristotle, there were three others:
Formal Cause answered the question, "What kind of thing is it?" It was the metaphysical pattern of a thing, its form.
Material Cause answered the question, "What is it composed of?" This was the actual matter that makes a thing up.
Final Cause answered the question, "What is it for?" This is a thing's purpose—for the Greeks, its telos.
Let's use an illustration: a building. The formal cause of the building is the blueprint on the basis of which it was constructed; the material cause is the material it was made of—lumber, nails, bricks, etc.; the efficient cause, that which brought it about, was the contractor and the carpenters and bricklayers; the final cause was the purpose of the building—to house a family (a house) or to provide a place for workers (office building) or to worship (a church).
If you could answer these four questions about a thing, then you could be sure you knew what it was. You had explained it.
We no longer use all of these causes, largely because of developments in philosophy and science over the last several hundred years. Formal and final cause were rejected by thinkers such as Francis Bacon and later materialist philosophers involved in the scientific revolution of the 17th and 18th centuries because they were considered metaphysical. A thing's nature or essence or form (formal cause) or its purpose (final cause) were considered by Aristotle to exist in things themselves. But the newer thinkers considered them to be subjectively imposed upon things from without, existing only in our minds.
Matter is still believed in, but not in the same way, and it is not really thought of as a cause.
All that is left is efficient cause. But this too has been called into question by some physicists. According to the Copenhagen interpretation of quantum mechanics (still considered the standard interpretation), the fundamental elements of which things are made do not follow the laws of cause and effect as we know them. Certain particles disappear and then reappear somewhere else instantaneously (a quantum leap), and subatomic particles do not exist anywhere until we observe them (said Neils Bohr).
According to Bohr, one of the pioneers of quantum mechanics, science is no longer in the explaining business at all. All it can do is predict.
But rather than accept the dead end of empirical science, there are still many philosophers think that Aristotle's four causes are far from obsolete.