This is the 9434th most frequent Japanese word.
積み
"積み" (tsumi) means "pile," "stack," or "accumulation."
Here, '積み込む' (tsumikomu) means 'to load,' representing the act of loading items into a vehicle or container.
彼は荷物をトラックに積み込んだ。
He loaded the luggage onto the truck.
In this sentence, '積み重なる' (tsumikasanaru) means 'to pile up,' indicating the stacking of objects one on another.
持っている本は積み重なっている。
The books I own are stacked up.
The term '積み重ねる' (tsumikasaneru) is used metaphorically here to mean 'to accumulate,' referring to the gathering of knowledge or experience over time.