It’s basically a Kubernetes “API object description”. A config file can include one or more of these. (i.e. Deployment, ConfigMap, Secret, DaemonSet, etc)
As per this:
Specification of a Kubernetes API object in JSON or YAML format.
A manifest specifies the desired state of an object that Kubernetes will maintain when you apply the manifest. Each configuration file can contain multiple manifests.
And a previous version of the documentation:
Configuration files – Written in YAML or JSON, these files describe the desired state of your application in terms of Kubernetes API objects. A file can include one or more API object descriptions (manifests).