This is the 732nd most frequent Persian word.
اش
"اش" (Āsh) is a Persian dish, typically a thick soup or stew made with herbs, vegetables, legumes, and sometimes meat or noodles.
The suffix "اش" means "his/her/their" here, attached to "کتاب" to imply possession.
کتابش را روی میز گذاشت.
They put their book on the table.
Here, "اش" serves as a pronoun, representing an object emphasized as "it."
اش را به خاطر بیاور.
Remember it.
In this example, "اش" represents "him/her" as part of the prepositional phrase indicating accompaniment.