A "View", also called a "Filter", is a specific subset of Accounts, Contacts, Deals or Leads created in Freshsales.
Freshsales populates all accounts with some default Views, and it also lets Freshsales clients create their own:
Often a View ID will be visible in your browser's address bar while you are looking at your View inside Freshsales. Look for a number in the URL.
Alternatively you may flatten your entire list of Views by running a Flatten Once job (will take less than 30 seconds) inside Flatly of the following:
Open up your flat file (in a CSV reader, a Google Sheets or Excel Online) and look at the ID column. Copy the ID to your computer's clipboard.
Finally, paste your View ID into the "Filter" setting in your job's Advanced Settings in Flatly. If you've correctly entered a valid View ID, Flatly will save it to your job.