1To pile or stack things up, one on top of another.
倉庫に段ボール箱をきれいに積んだ。
I neatly stacked the cardboard boxes in the warehouse.
子どもが積み木を高く積んで遊んでいる。
The child is playing by stacking building blocks high.
古新聞を紐でまとめて玄関に積んでおいた。
I tied up the old newspapers with string and stacked them by the entrance.
2To load cargo or luggage onto a vehicle, ship, or truck.
引っ越しの荷物をトラックいっぱいに積んだ。
We loaded the moving truck full of belongings.
漁船に大量の魚を積んで港に戻ってきた。
The fishing boat came back to port loaded with a huge catch of fish.
出発前にスーツケースを車のトランクに積んだ。
Before setting off, I loaded the suitcases into the car's trunk.
3To gradually accumulate experience, knowledge, or achievements over time.
彼はこの分野で長年経験を積んできた。
He has built up years of experience in this field.
若手のうちに場数を積んでおくと、後で役に立つ。
Getting plenty of practical experience while you're young will help you later on.
選手たちは大会に向けて練習を積んでいる。
The athletes are building up their training for the competition.