yaozhao It means people have clicked on the link you sent out but no one has persisted all the way through to completion. It sounds likely that your survey is too taxing on respondents (especially since someone took the time to message you about it). In the data and analysis tab, you can look to see the Responses in Progress - likely 263. This table also shows you where they left off and you can see if they are hitting a sticking point. See this information page for more detail on reviewing the responses in progress
https://www.qualtrics.com/support/survey-platform/data-and-analysis-module/data/responses-in-progress/

Yes, see https://www.qualtrics.com/support/survey-platform/data-and-analysis-module/data/responses-in-progress/

Thank you. In the distribution tab, it says "287 Surveys Started
267 Surveys Finished"

But when I click Data & Analysis tab, it says "Responses in progress 20". I am wondering why there's a difference between 287 and 20.

Because if 287 people started the survey, and 267 of them have finished, that leaves 20 in progress. Note that these numbers won't always add up with each other, though, depending on how long your survey is open and the partial complete settings you've applied (record or delete).

267 + 20 = 287
267 is the number of completed surveys
20 is the number of started but not completed surveys
287 is the total number of started (in progress or completed) surveys