Tiller has been removed in Helm 3.
The Tiller server is an in-cluster component that interacts with the Helm client and interfaces with the Kubernetes API Server.
The server is responsible for managing releases. It does that by handling client requests, combining a chart and the configuration to build a release, installing charts and then tracking their subsequent release, then upgrading and uninstalling charts.
Tiller stores release information in ConfigMaps in the namespace where it is running. It could be configured to use a storage backend that uses Secrets for storing release information. More here. It could also be configured to use an SQL storage backend for storing release information. More here.