On a Survey Options page, in Security Options section, there is a "Restrict Survey with Access Codes" setting. If you switch that setting to ON, then your survey will ask the respondents for their access code prior to letting them take the survey.
If this setting is turned ON, you will also see a slot where you can add access codes for this survey, one access code per line. All access codes that you enter here will be used as a whitelist for survey takers. Every respondent will have to provide a valid (specified here) access code in order to take a survey.
If you wish to keep your survey open and accept all respondents, you can use access codes to gather info on your respondents too. By checking the "Add Access Codes on the Fly" setting, every entered access code will be automatically added to the above-mentioned list of access codes. This is perfect if you wish to ask your respondents for their email address (to act as an access code). This way the whitelist is not used for evaluating entered access codes, whoever takes the survey will have their access code added.
