You have different options to do that. Here are the most popular ones:
(1) If you invite respondents via mailing (individual links sent via SoSci Survey), then you can resume answering with the individual link at any time. Even if the browser window was closed accidently.
(2) In Compose Questionnaire -> Settings you can enable a button to pause the interview. This button will offer different options to resume.
Technically, the data is stored whenever the respondent clicks "next". The challenge is to identify the correct respondent when they come back to the survey.
Further options are a cookie and multi-wave surveys.