Space-time is a concept with which we can attempt to understand the nature of our Universe.
Everything in the Universe needs four dimensions to "place" it: three dimensions within space (up, down and across, giving "where" it is) and a fourth dimension (time), giving the "when".
For example, reading this, you can place yourself "in" space, but you are also reading it in a particular "now". Stand up, or sit down and you've changed your position in space, but time has also changed: it will be a different "now".
The two properties of space and time are inextricably connected with each other, into space-time, because absolutely everything is moving, relative to everything else in the Universe.
The velocity of light, however, is unchanging: if we measured it whilst ourselves travelling in an unbelievably fast space rocket - say at half the velocity of light - it would be the same as measuring it on Earth. But, it "shouldn't" be so: we would be moving superfast, too, so the velocity of light we measure whilst on the rocket should be slower, because we are "catching up with it" - a bit like running after a train - not just sitting around on the Earth, waiting for the train (light ray) to come to us.
Velocity is distance travelled in a given amount of time. The only explanation for the velocity of light being experienced the same on the rocket as on the Earth is if the metres and the seconds have changed, and this is actually true. The faster something moves in space, the slower time will pass, and the shorter metres will become, relative to a stationary observer, until, if we ever reached the speed of light, distances would become zero and time would completely.....stop(!)