These are your survey instructions that you would enter for your survey participants. You may put whatever text you like here, which may include information about the purpose of the survey, who is taking the survey, or how to take the survey.
Surveys can use a single survey link for all respondents, which can be posted on a webpage or emailed out from your email application of choice. By default, all survey responses are collected anonymously (that is, unless your survey asks for name, email, or other identifying information). If you wish to track individuals who have taken your survey, you may upload a list of email addresses into a Participant List within REDCap, in which you can have REDCap send them an email invitation, which will track if they have taken the survey and when it was taken. This method still collects responses anonymously, but if you wish to identify an individual respondent's answers, you may do so by also providing an Identifier in your Participant List. Of course, in that case you may want to inform your respondents in your survey's instructions that their responses are not being collected anonymously and can thus be traced back to them.