A soft launch refers to gradually sending out email invitations for a survey so that any teething issues can be addressed before all of your contacts are invited.
This is not a replacement for testing your survey thoroughly first. It is an additional precaution you can take that can be a real life-saver in case your testing missed something. You often only have one shot with respondents. If they can, for some reason, not complete your survey, they probably will not try again later.
Some of the common issues that can be identified with a soft launch are things like response options that are missing. For instance, a simplified example, you ask the respondent to select their country and it is not in the list or you set up some branching based on a custom field, which is not working for some of the contacts.