The Gardener Seed admission controller is deployed by the Gardenlet as part of its seed bootstrapping phase and, consequently, running in every seed cluster. It's main purpose is to serve webhooks (validating or mutating) in order to admit or deny certain requests to the seed's API server.
As part of Gardener's extensibility concepts a lot of CustomResourceDefinitions are deployed to the seed clusters that serve as extension points for provider-specific controllers.
For example, the Infrastructure CRD triggers the provider extension to prepare the IaaS infrastructure of the underlying cloud provider for a to-be-created shoot cluster.
Consequently, these extension CRDs have a lot of power and control large portions of the end-user's shoot cluster.
Accidental or undesired deletions of those resource can cause tremendous and hard-to-recover-from outages and should be prevented.
Together with the deployment of the Gardener seed admission controller a ValidatingWebhookConfiguration for CustomResourceDefinitions and most (custom) resources in the extensions.gardener.cloud/v1alpha1 API group is registered.
It prevents DELETE requests for those CustomResourceDefinitions labeled with gardener.cloud/deletion-protected=true, and for all mentioned custom resources if they were not previously annotated with the confirmation.gardener.cloud/deletion=true.
This prevents that undesired kubectl delete <...> requests are accepted.
The admission controller endpoint /webhooks/default-pod-scheduler-name/gardener-kube-scheduler mutates pods and adds gardener-kube-scheduler to .spec.scheduleName.
When SeedKubeScheduler feature gate is enabled, all control plane components are mutated. The scheduler scores Nodes with most resource usage higher than the rest, resulting in greater resource utilization.