Polling vs Push
Two ways to get new data: polling repeatedly checks a source on a schedule, while push (webhooks) has the source send data the instant an event occurs.
Analogy
Polling is refreshing your inbox every minute; push is getting a notification the second mail arrives.
Why it matters
Choosing push over polling makes automations faster and cheaper, since polling consumes operations/tasks on every check whether or not data changed.
In practice
A Zapier polling trigger checks for new rows every 15 minutes, while a webhook trigger fires immediately.