By default, all questions that you add to your survey will be optional. In other words, survey respondents will not be required to answer the questions. If questions are made mandatory, then the respondent will get an error that will not allow them to submit the question unless they have answered it fully.
How to Use:
In the options tab for every question type you will see some checkboxes that you can use to specify whether a question will be mandatory or not. For matrix questions, you can specify whether a question is optional or not at the variable level.
The video below includes some relevant information on optional/mandatory questions:
More information here.