"Web Data" is a list of sites that you've visited, links to sites that you've visited that were opened, and associated content including cookies. This allows Safari to load these sites faster the next time they're visited as a majority of the content is usually static, so Safari only downloads and displays any data that's changed. You could delete this data without any negative impact...other than those same sites loading a little slower the next time you visit them.
You can delete the data from within the Advanced area or in Safari's settings by tapping "Clear Cookies and Data".