one's younger days; when one was young
A literary or nostalgic expression referring to one's youth, often used in reminiscence. The かりし is a classical adjectival form of 若い.
We shared memories of our younger days.
I heard that my grandfather was a sailor when he was young.
Kana spelling of 頃; slightly less formal but common.
From 若い (young) in its classical adjectival form 若かりし (attributive of 若かりき) + 頃 (time, period). The phrase is a fixed literary expression.