A Student ID is a unique identifier assigned to you by your school. Note that your college or university may call it something else, such as netID or eID. Your Sapling Learning profile has a field called ID Number (which may contain letters and/or numbers) that is meant to hold your university Student ID. When filled in correctly, it allows your instructor to more easily transfer your homework grades to their local grade system.
At any time, you may add or edit your Student ID (also known as ID Number) by editing your profile.
Additionally, your instructor may have chosen a setting that prompts you to enter your university Student ID, if missing, when you view the course page. In these cases, you won't be able to work on the assignments until you enter your ID into the popup and click Ok. Once entered, you will not be prompted again.