Explaining all there is to know about the Engine would be impossible in a simple answer, however, I can recommend you start with the introduction part of the documentation. It can be a bit wordy, but should explain properly what you need to know. The documentation is also the main place to find information about Godot.
If you prefer discovering with examples, you can download an example project from the AssetLibrary (also available on the project creation screen). You will still end up in the documentation at some point, but it might help you stay motivated.