wooden building blocks; toy blocks
Refers to the physical wooden blocks used as a children's toy, or the activity of playing with them.
The child is playing with building blocks.
piled timber; stacked wood
Refers to logs or lumber stacked up, not toy blocks. Less common than the toy sense.
Piled timber was left at the foot of the mountain.
積み木を高く積み上げた。
I stacked the blocks high.
Standard spelling; the okurigana み makes the reading clear.
Less common variant without okurigana; may be encountered but 積み木 is preferred.
Compound of 積み (tsumi, 'piling up', the continuative form of 積む) + 木 (ki, 'wood'). Literally 'piling-up wood'.