This setting allows a participant to save the responses to a survey he is taking at the moment and complete the survey at a later time using a username and password.
Normally, this setting is "On" by default. If you want to double-check this setting or turn it off you can navigate there inside the application by:
Configuration (from the main navigation in the header of the application) -> Global survey (where you can set global survey settings) -> Notifications and data -> Participant may save and resume later (enable/ disable this setting by toggling On / Off).
In case you have this setting activated ("On"), your survey results on the "Responses" screen from the survey menu will also show you a button labeled with "View saved but not submitted responses". This way you can check the responses from participants who have started taking you survey, but want to return later to take it until the end.
Navigate there by:
Surveys (main navigation in the header of the application) -> Select a survey (it needs to be active to be able to show responses) -> Responses (from survey menu) -> View saved but not submitted responses.