Relativity in brief... or in detail..
Module 1. Relativity from Galileo to Einstein
Relativity began with Galileo, who said that, in the laws of mechanics, only relative motion is important. In other words, mechanical forces are the same for the person on the train and the platform. All they need do to convert their measurements is to add or subtract the relative velocity.
There is no way that a person in a smoothly moving ship can tell whether the ship is moving or not.
If you can juggle on the ground, you can also juggle on a smoothly moving ship, a train or even a plane: the laws of physics the forces involved are the same, even if a bystander sees juggler and balls all whizzing past at high speed.