The article explains how to onboard an existing Helm application into ArgoCD, a GitOps tool for continuous deployment in cloud-native applications. To start, the user needs a Kubernetes cluster and Helm v3 installed, as well as a remote Helm repository containing the application. The user can install the app using the Helm repository by adding the repo, installing the chart, and running `kubectl get pod` to verify the pods are running. Next, the user installs ArgoCD into their cluster and creates an Application manifest for the app, specifying the source repository, target revision, and helm values. The user applies the manifest using `kubectl apply -f`, and then syncs the application using `kubectl get all -l`. Alternatively, if the Helm chart is stored in a Git repository like GitHub or GitLab, the user can create an Application manifest pointing to the repository and path of the helm chart. Once onboarded, the app will be out of sync with annotation changes until the user reviews and syncs it using `kubectl get all -l`. The article also mentions that certain applications may require additional setup, such as adding a checksum or secret annotation.