By default, ethnio sets a cookie to prevent anyone from ever seeing a screener twice within 30 days for a given domain. But often for testing, it's easier to disable the cookies so you can test a screener a bunch without having to clear your cookies every time. Otherwise, you probably always want to leave this on. Keep in mind it also applies to direct links, so people can't fill out your screener multiple times. Here's some technical information about the cookie:
Cookie Name: _ethnio_displayed
Purpose: This cookie is set once a visitor sees an Ethnio screener on a given domain (for ex, customer.com) to prevent that visitor from seeing another Ethnio screener on that domain for 30 days. The main purpose of this cookie is to make sure site visitors aren’t annoyed by multiple screener invites. You can disable cookies under Screeners >> Publish
Duration: 30 days
This setting is under screeners >> publish.