The tabs available on the Respondents list page are there to show you the progress status of each respondent. ANSWERED refers to the number of respondents who have answered one or more questions. COMPLETED  refers to the number of respondents who have completed the survey. ACCESSED refers to respondents that have clicked the survey link but have not answered any questions yet. Finally, NOT ACCESSED refers to those respondents who have not clicked the link to enter the survey.
Keep in mind that each question in the survey will display an answer frequency, which will help you to understand if any respondents have dropped-off in the survey. if you want to know more about drop-offs in your survey, you can use the Drop-off frequency per question function under the modules tab.