What is "spacetime" and how does it "curve"?

No.13440384 ViewReplyOriginalReport
Any time my small brain tries to understand general relativity it hurts. There's nothing physical to curve, right? Does the implication that spacetime exists as something to be "curved" imply that our universe is the surface of a 4D object? What does it mean? What's it curving into? How does mass create curvature in the first place? How does gravity result from this?