several days; a period of a few days
Refers to a span of roughly 2–6 days. Often used when the exact number is unknown or unimportant.
It kept raining for several days.
He took a few days off.
数ヶ月 means 'several months', a longer time span.
Compound of 数 (several) + 日 (day) + 間 (interval/duration).