a day or two; one or two days
Used to indicate a short, indefinite period of a couple of days. Often appears in contexts where the exact number of days is not important or is approximate.
If it's a cold, you'll get better after resting for a day or two.
This job isn't something that can be finished in a day or two.
数日 means 'a few days' and is more formal and vague. 一日二日 is more colloquial and explicitly limits the period to one or two days.
Compound of 一日 (one day) and 二日 (two days), literally 'one day, two days'. The reading いちにちふつか follows the standard readings of the individual components.