Rarely stands alone in modern Chinese; appears in compounds: 主旨 ("main idea / thesis"), 宗旨 ("aim / purpose"), 圣旨 ("imperial decree"), 要旨 ("essential meaning"). Memorize via these collocations rather than as a solo word.
日 here is not "sun" - it stands in for an older 甘 (sweet) element, a mouth-with-something-inside drawn as a flat square. Modern dictionaries file 旨 under 日 because of the shape, not the etymology.
匕 on top is the old spoon graph. 旨 once meant "tasty" - food brought to the mouth with a spoon was something worth savouring, and from "tasty" the sense drifted to "the gist worth taking in," then "imperial wish."
