Closed Access Mode adds a level of security and control to your questionnaire(s). A participant must provide a valid token in order to submit responses to a questionnaire in Closed Access mode. Closed Access mode is similar to a password protected questionnaire without requiring that you create full user accounts for each questionnaire participant. Closed Access mode is recommended for all questionnaires available on the publicly accessible Internet. Questionnaires hosted on a private, local network may not need Closed Access mode enabled, but ultimately the decision should be dependant upon your unique requirements.
Steps 10-15 in CONFIGURATION detail one method of enabling Closed Access Mode and enabling Tokens.
The instructions here explain the basics of configuring Closed Access mode.
For a full description of all the functionality available related to Closed Access mode, see http://manual.limesurvey.org/Tokens
Copies of LimeSurvey webpages in the instructions can be enlarged by left-clicking the copy of the webpage.