[convert the strings to base64]
$ echo -n 'admin' | base64
YWRtaW4=
$ echo -n '1f2d1e2e67df' | base64
MWYyZDFlMmU2N2Rm

[Opaque]
It is the default Secret type.
$ kubectl create secret generic empty-secret

$ kubectl get secret empty-secret
NAME           TYPE     DATA   AGE
empty-secret   Opaque   0      2m6s

In above, the 'DATA' column shows the number of data items stored in the Secret.
In this case, '0' means you have created an empty Secret.

[Docker]
$ kubectl create secret docker-registry secret-tiger-docker --docker-email=tiger@acme.example --docker-username=tiger --docker-password=pass1234 --docker-server=my-registry.example:5000

$ kubectl get secret secret-tiger-docker -o jsonpath='{.data.*}' | base64 -d
{
  "auths": {
    "my-registry.example:5000": {
      "username": "tiger",
      "password": "pass1234",
      "email": "tiger@acme.example",
      "auth": "dGlnZXI6cGFzczEyMzQ="
    }
  }
}

[TLS]
$ kubectl create secret tls my-tls-secret --cert=path/to/cert/file --key=path/to/key/file

[Opaque]
$ kubectl create secret generic ssh-key-secret --from-file=ssh-privatekey=/path/to/.ssh/id_rsa --from-file=ssh-publickey=/path/to/.ssh/id_rsa.pub
