Hannah praises the LORD because He gave her a son. She was barren and taunted by “her rival” (v. 6) but she prayed desperately for a son.
A prophet (1Sa 3:20) and effectively the last judge of Israel (Acts 13:20). He appoints the first king of Israel… then condemns him as unfaithful to God. Then Samuel appoints the successor in waiting.
The main idea is that the firstborn — especially a son — gets “dibs” on everything, like the inheritance, or at least the larger share of it. But there is more to the word than just the idea of “the first son to be born.”
The book of 1 Samuel, which is about David and Saul. And Samuel.