A webhook is an HTTPS request the gateway sends to your endpoint when something happens (a message is delivered or fails, a reply arrives, the balance runs low), so you do not have to poll for status.
Instead of asking the API repeatedly whether a message has been delivered, you register an endpoint and receive an event when it is. Events carry an id, a type, a timestamp and the data; they are signed so you can verify their origin, and they are retried when your endpoint does not answer.
A production handler verifies the signature, acknowledges within seconds, processes from a queue, deduplicates on the event id and orders by time, because retries and replays deliver the same event more than once and not always in order.
At SMSMeteor: events are signed with HMAC-SHA256 in X-Meteor-Signature, retried 6 attempts over 24 hours, and replayable from the panel. See the webhooks page.