Zapier only sends data from your trigger app that was created after turning the Zap on. So your Zap will not be able to see data that was created before your Zap was turned on. If you’d like to trigger on existing records or import existing data, there's a workaround you can use.
Say you created a Zap to take every new signup to a Mailchimp list and add them to Salesforce. This Zap would not move everyone who had already signed up on Mailchimp to Salesforce, only those who signed up after turning on the Zap.