Sync asks Klaviyo which Sprwt metrics already exist in your account, and for every event type that is missing it sends one sample event (using your own admin profile and sample data) so the metric is created. This is what makes triggers such as order_create, user_subscriptions_skip or order_abandoned_cart selectable when you build a flow in Klaviyo. Metrics that already exist are left untouched, so you can click Sync as often as you like.

Sync does not upload your customer lists - list syncing to Klaviyo happens on its own schedule (see the question about lists). If nothing appears in Klaviyo after Sync, double-check both keys and that the Klaviyo module is enabled; errors from Klaviyo are not shown on the page.