What is meant by space time? Can you please explain in layman's terms?

The essence of it is that time is a dimension, exactly like the three dimensions of space. Humans perceive time as being something different, only because their brains are designed to do so. (Actually they have to be, in order to conform to the laws of thermodynamics). So, instead of thinking of 3-D space and time as two seperate things, you have to consider that the whole thing is 4-D space-time. It is impossible for a human to visualise 4 dimensions simultaneously, so the only sensible way to think about it is to ignore one of the 3 space dimensions, and think of it as the time dimension instead. Then people represent 'life-lines', which trace a route through the time dimension. If they spend their whole lives immobile, then the line will be straight. If they move around, the line traced out will 'wiggle', due to movement in the two spatial axes perpendicular to the time axis.