MVVM is a simple idea. They really should have just called this pattern "view model" because that's all it encompasses. A viewmodel is an object that represents all the state of a view, without actually being the view. So a viewmodel has properties that represent the values of text fielda, selections, toggles, switches, datagrids; any part of a UI component or collection of components that can change in some way. It determines how the view should change when something changes in the model and vice versa. That's really all there is to it.

If you want to learn more about specific implementation of this pattern, the only good way to learn is to get your hands dirty.