To pile up; to accumulate (intransitive) — things accumulate or stack up one upon another, including physical objects or abstract things like experience.
机の上に請求書が積み重なっている。
Bills are piled up on the desk.
雪が何日も降り続いて屋根に積み重なった。
Snow fell for days and piled up on the roof.
少しずつ経験が積み重なって、仕事が早くなった。
Experience accumulated little by little, and I got faster at the job.