I am looking at https://godoc.org/k8s.io/api/core/v1#Secret

type Secret struct {
    metav1.TypeMeta `json:",inline"`
    // Standard object's metadata.
    // More info: https://git.k8s.io/community/contributors/devel/api-conventions.md#metadata
    // +optional
    metav1.ObjectMeta `json:"metadata,omitempty" protobuf:"bytes,1,opt,name=metadata"`

    // Data contains the secret data. Each key must consist of alphanumeric
    // characters, '-', '_' or '.'. The serialized form of the secret data is a
    // base64 encoded string, representing the arbitrary (possibly non-string)
    // data value here. Described in https://tools.ietf.org/html/rfc4648#section-4
    // +optional
    Data map[string][]byte `json:"data,omitempty" protobuf:"bytes,2,rep,name=data"`

    // stringData allows specifying non-binary secret data in string form.
    // It is provided as a write-only convenience method.
    // All keys and values are merged into the data field on write, overwriting any existing values.
    // It is never output when reading from the API.
    // +k8s:conversion-gen=false
    // +optional
    StringData map[string]string `json:"stringData,omitempty" protobuf:"bytes,4,rep,name=stringData"`

    // Used to facilitate programmatic handling of secret data.
    // +optional
    Type SecretType `json:"type,omitempty" protobuf:"bytes,3,opt,name=type,casttype=SecretType"`
}

Take Data map[string][]byte `json:"data,omitempty" protobuf:"bytes,2,rep,name=data"` for example, I know Data is the name and map[string][]byte is the type, what is the third thing that follows? What does it do and when is it necessary to include this third thing?