Progress Bar denotes the percentage of Survey loaded.
For Example: Suppose you have a Survey with 2 questions and there is a page break after Question 1. When respondents open the survey it will display that 50% of the Survey has been loaded. When respondents answer the question on that page and click on the Continue button the progress bar will display 100% as now the respondent is on the last page of the survey.
For single page surveys the progress bar will always display 100% loaded.