Components Managers implement the APIs that are used by Components. For example, they collect information about the user device, or client-side events, and they pass them to the Components. Components can then respond with different asks - make a network request, save client-side information, load a widget and more - and the Manager then acts upon these asks.
The configuration for Managed Components is also done through the Components Manager. It determines which Components to load, when, what permissions to give them, and what configuration should be passed to them.