The word 好几天 is formed by combining the character 好, which here functions as an adverb meaning "quite" or "very", with 几, meaning "several", and 天, meaning "day". The adverb 好 intensifies the quantity indicated by 几, and this modified quantity is directly applied to 天, resulting in a phrase that literally means "quite several days" or "several days" with an emphasized, indefinite plural sense.