The Meta Descriptions
Having a meta description tag in your page helps when a relevant snippet can't be found on the page. The meta description report helps you find where these are missing, duplicated, or where there are issues.

Source URL
Like all reports, this is the url of the page the description is found on.
Description
This is the text extracted from the meta description on the page. It's handy to sort these, for example, to look for duplicates or to use the "Filter text..." filter to look for keywords being used.
Issues
This will list any issues the crawler found. Because it's quite subjective, this doesn't focus on things like length but things like having two meta descriptions on a page.