It is not as precise as "every other day" or "every second day", but it means once in a two day period.
For instance, you have a grandmother who lives in a nursing home. You go to visit her every two days. This week you go on Sunday, and then again on Tuesday. Your next visit should be on Thursday, but you have an appointment on Thursday, so you go on Wednesday instead. Then maybe you go on Friday, or wait until Saturday. You are going once in every two day period.