In the context of Kubernetes, what is a mutating webhook?
They are an API invocation that make changes to resources (and in some cases before they are created per medium.com).
"MutatingAdmissionWebhook is one of easiest ways of extending Kubernetes with new policy controls and resources mutation and a host of other options." (Taken from medium.com )
"Mutating controllers may modify related objects to the requests they admit; validating controllers may not." This quote was taken from this website.
If you want to build a mutating (admission) webhook, see this external posting.
Unlike a validation webhook, mutating webhooks happen at a different step in the admission controller process. The admission controller process starts with an API request and ends with persisting data to etcd.
See the diagram here and visually search for "diagram representing."
Object schema validation precedes validation webhooks; object schema validation happens after mutating webhooks.