This is the 18237th most frequent Korean word.
쉬었다
"쉬었다" means "rested" or "took a break."
In this usage, '쉬었다' denotes the act of resting in reaction to a previously completed event – arriving at the mountain top.
우리는 산 정상에 도착한 후 얼른 쉬었다.
We quickly rested after arriving at the mountain top.
Here, '쉬었다' is used to imply the act of resting, but the main sentence illustrates a feeling of insignificance of that rest due to being busy.
쉬었다고 느껴지지 않을 정도로 바빴다.
I was so busy that it didn't even feel like I rested.
Here, '쉬었다' is the past tense of '쉬다', meaning 'to rest', indicating the completion of the action of taking a break.