What is relativity?

You ask: What relativity?

Long-ago thinker named Einstein see truth: light always fly same speed, no matter how fast Caveman chase it — light race round whole Earth near seven times in one heartbeat. Strange! To keep light speed same for all, time and space must stretch and squish. Caveman run near light-fast — Caveman clock tick slow, Caveman body squash short. That special part. Big part: heavy things bend space itself. Think mammoth sit on stretched hide — hide dip down. Small rock nearby roll into dip. Caveman call that dip 'gravity.' Sun make big dip, so Earth roll around it. Not rope pulling — just Earth follow bent ground. Even time run slow near heavy thing. Cave wisdom: space and time not fixed cave walls — they bend, stretch, and lean on heavy stuff.