SpaceTime
Einstein's equations joined space and time into a single continuum.
This was different to Newton's picture of space and time. Newton's equations described matter moving through space from one moment to the next. Time was thought to flow in the same way in all parts of the Universe. Any moment in time could be considered like the frame of a giant 3d movie. Most people think about time like this.
Unfortunately this idea of time and space ran into difficulties. To solve these difficulties, Einstein had to change this picture.
Einstein showed that time could not be considered separately from space—they had to be considered together. Einstein's equations were not written in three dimensions of space, like Newton's, but in four dimensions of spacetime.
Moreover, the way time flowed, or the way that space appeared could change, depending on the location and motion of the observer. Different observers could disagree about the lengths of objects, the time between events, or even the order in which some events happened!
To an astronaut on the Moon, a spaceship travelling near the speed of light towards looks shorter. To the rocket it is the Moon that is moving and appears squashed.
