How does Aristotle define "first principles" in his 'Metaphysics'?

Ha, that's an interesting question. If I were to explain Aristotle's "First Principles" to an ordinary person, I think I'd put it this way:

You can imagine it like playing the "getting to the bottom of it" game. For example, you ask a child, "Why do you go to school?" They say, "To learn knowledge." You ask again, "Why learn knowledge?" They say, "To get a good job in the future." You continue, "Why get a good job?" They say, "To earn money and support my family." You still ask, "Why earn money and support your family?"...

If you keep asking like that, eventually you'll reach an answer that they feel is "self-evident and needs no further explanation," such as "to survive" or "to be happy." This "self-evident" ultimate answer, which cannot be questioned further, is what Aristotle called a "First Principle."

From Aristotle's perspective, it has several characteristics:

1. It's the foundational cornerstone: Just as a house needs a foundation, any knowledge or theory must have a most fundamental starting point. This starting point is the First Principle.
2. It is self-evident: You don't need to use other things to prove it, because it is the most fundamental "fact." For instance, in geometry, "the shortest distance between two points is a straight line" is an axiom; you don't prove it, but rather use it to prove other, more complex theorems. A First Principle plays a similar role to an "axiom."
3. It is the source of all deductions: Once you find this most fundamental cornerstone, you can start from it and logically derive an entire system of knowledge step by step. Because your starting point is solid and reliable, subsequent deductions will also be sound.

So, simply put, Aristotle's "First Principle" is that most fundamental, self-evident starting point within any system or theory. Finding it is like finding the key to unlock the entire system.