When setting up your survey, part of the setup is how you handle your respondents and their survey links / passwords.
If you hit the Key / Access button at the top, you can define a password field and import as many passwords as you desire. Click on the Password Data button at the bottom and you will be able to data enter, import or auto generate your passwords.
If each password has the Maximum Respondents set to "1", then that password will only allow one survey to be completed.
If a respondent suspends their survey using a unique password, they can re-click on their survey link and they will commence back at the suspension point, which is what you want.
If you use generic passwords where respondents can complete the survey many times for any given password, you can click on the Cookie-based Restart tick box under the Settings tab. This will convert (in a sense) a generic password to a unique password, providing the respondent allows cookies on their device. If they don't, the password will act like a generic password. It is a handy feature which I like to use.
I suggest you generate about 10 unique passwords and give it a try. Then create one generic password (set the Maximum Respondents to 100) and test it out with and without the Cookies-based Restart check box being ticked.
This will be a good exercise into the understanding of how your survey links / passwords behave.