PBS
Pbs Nova: Relativity and the Cosmos
This well-written explanation of relativity includes animations and pictures which may be helpful in understanding a tough topic.
Science Struck
Science Struck: What Is Gravity and How Does It Work
A very detailed look at gravitational force and the many theories that have been put forward to try to explain it. Includes lots of illustrations.
Libre Text
Libre Texts: Physics: A Geometrical Theory of Spacetime
Understand the details of Einstein's relativity with the assistance of this detailed resource.
University of St. Andrews (UK)
University of St. Andrews: General Relativity
A somewhat labored but thorough explanation of general relativity, beginning with Aristotle. Many links to the personalities connected to the article.