The Template Controller is a controller originating from the Kluctl project, but not limited to Kluctl. It allows to define template objects which are rendered and applied into the cluster based on an input matrix.
In its easiest form, an ObjectTemplate takes one input object (e.g. a ConfigMap) and creates another object
(e.g. a Secret) which is then applied into the cluster.
The Template Controller also offers CRDs which allow to query external resources (e.g. GitHub Pull Requests) which can
then be used as inputs into ObjectTemplates.
Template Controller has many use case. Some are for example:
Reference documentation is available here.
The announcement blog post also contains valuable explanations and examples.
Installation instructions can be found here