Libre Text
Libre Texts: Physics: Introducing General Relativity
Explains what is meant by Einstein's Theory of General Relativity.
PBS
Pbs Learning Media: Einstein: How Smart Was He?
This essay from the NOVA Web site explores the impact Einstein made on physics and most everything we know about the cosmos.
Other
Uc Riverside: Twin Paradox
To make sense of this, you have to first see the Twin Paradox: Introduction, which is the special relativity version and is linked to this page. This general relativity explanation views the paradox from a gravitational point of view.