The installation is broken up into two different use cases: Single and Multi-Cluster install. The single cluster install is for if you wish to use GitOps to manage a single cluster, in which case you do not need a centralized manager cluster. In the multi-cluster use case you will setup a centralized manager cluster to which you can register clusters.
If you are just learning Fleet the single cluster install is the recommended starting point. After which you can move from single cluster to multi-cluster setup down the line.