Your firstborn is your first child — the one born before any others. It works as a noun ('their firstborn', 'she was the firstborn') and as an adjective before a noun ('his firstborn son'). Plain English would just say 'eldest child' or 'oldest son', and in ordinary conversation that is what people say. 'Firstborn' is heavier. It carries a lot of old weight, because for centuries in many places the firstborn son inherited the land, the title, the family name. So the word does not just record an order of birth; it hints at rights and duties. You meet it a great deal in the Bible, in stories, in history, and in formal or emotional writing. That gives it a slightly grand, sometimes solemn sound. A parent might say 'my firstborn' fondly and half-jokingly about a grown-up child, and everyone hears the warmth in it. But you would not normally ask a stranger 'Is this your firstborn?' — 'Is she your oldest?' is what a real person says. Note the spelling: it is normally one word, firstborn, though you will see first-born with a hyphen in older books and it is not wrong.