MeaningA word that connects two ideas, like "that", "which", or "who". It points to a person or thing you just mentioned to say more about it.
HistoryThis is a very old word from the Bible. In ancient times, it was the main word used to connect ideas, meaning "that" or "which". Today, people usually say a shorter word, "שֶׁ", but you will still see "אֲשֶׁר" in books, official papers, and songs.
Exampleזה הספר אשר קראתי
This is the book that I read.