Surname
Family Name
noun, の adjective
A seedling letter is the very first letter you learn when you're little. What's the first thing you learn to spell when you're little? Your surname! You need to know who your family is, otherwise if you get lost or something happens, you won't know how to get home! Or even who you are!
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
その苗字、初めて聞きました。
That was the first time I’d heard that surname.