This is the 2435th most frequent German word.
reihen
"Reihen" means "to arrange" or "to line up."
In this sentence, 'reihe' means 'I arrange' and it refers to the act of placing the books in a specific order according to size.
Ich reihe die Bücher nach Größe ins Regal.
I arrange the books in the shelf by size.
Here, 'reihen' describes the positioning of beads in succession on the necklace.
Die Perlen reihen sich perfekt auf der Kette.
The beads align perfectly on the necklace.
In this example, 'reiht sich' means 'joins' and it implies placing oneself in the queue with others in an orderly manner.